usability improvements ith how base options are registered

This adds named parameters for all of the current array-index based
options. It also allows specifying the description as the 2nd parameter,
allowing the 'required' parameter to be implicitly false (the most
common value).

A simple parameter like:

 OptAddress.new('ReverseListenerBindAddress',
   [false, 'The specific IP address to bind to on the local system']),

Can now be rewritten as:

 OptAddress.new('ReverseListenerBindAddress',
   'The specific IP address to bind to on the local system'),

More complex options are also now easier to read:

 OptString.new(
   'HttpUserAgent',
   'The user-agent that the payload should use',
   default: Rex::UserAgent.shortest,
   aliases: ['MeterpreterUserAgent']
 ),

This also makes dealing with enums easier because default is implicit
unless specified. This:

  OptEnum.new('PayloadProxyType',
    [true, 'The proxy type, HTTP or SOCKS', 'HTTP', ['HTTP', 'SOCKS']]),

Becomes:

  OptEnum.new('HttpProxyType',
    'The proxy type, HTTP or SOCKS', required: true, enums: ['HTTP', 'SOCKS'])

This maintains full backward compatibility with existing code as well.

lib/msf/core/opt_base.rb

@@ -22,15 +22,31 @@ module Msf
     # attrs[3] = possible enum values
     # attrs[4] = Regex to validate the option
     #
-    def initialize(in_name, attrs = [], aliases: [])
+    # Attrs can also be specified explicitly via named parameters, or attrs can
+    # also be a string as standin for the required description field.
+    #
+    def initialize(in_name, attrs = [],
+                   required: false, desc: nil, default: nil, enums: [], regex: nil, aliases: [])
       self.name     = in_name
       self.advanced = false
       self.evasion  = false
-      self.required = attrs[0] || false
-      self.desc     = attrs[1]
-      self.default  = attrs[2]
-      self.enums    = [ *(attrs[3]) ].map { |x| x.to_s }
-      regex_temp    = attrs[4] || nil
+      self.aliases  = aliases
+
+      if attrs.is_a?(String) || attrs.length == 0
+        self.required = required
+        self.desc     = attrs.is_a?(String) ? attrs : desc
+        self.enums    = [ *(enums) ].map { |x| x.to_s }
+        self.default  = default || enums[0]
+        regex_temp    = regex
+      else
+        self.required = attrs[0] || required
+        self.desc     = attrs[1] || desc
+        self.default  = attrs[2] || default
+        self.enums    = attrs[3] || enums
+        self.enums    = [ *(self.enums) ].map { |x| x.to_s }
+        regex_temp    = attrs[4] || regex
+      end
+
       if regex_temp
         # convert to string
         regex_temp = regex_temp.to_s if regex_temp.is_a? Regexp
@@ -45,35 +61,34 @@ module Msf
           raise("Invalid Regex #{regex_temp}: #{e}")
         end
       end
-      self.aliases = aliases
     end
 
     #
     # Returns true if this is a required option.
     #
     def required?
-      return required
+      required
     end
 
     #
     # Returns true if this is an advanced option.
     #
     def advanced?
-      return advanced
+      advanced
     end
 
     #
     # Returns true if this is an evasion option.
     #
     def evasion?
-      return evasion
+      evasion
     end
 
     #
     # Returns true if the supplied type is equivalent to this option's type.
     #
     def type?(in_type)
-      return (type == in_type)
+      type == in_type
     end
 
     #
@@ -94,7 +109,7 @@ module Msf
       if regex
         return !!value.match(regex)
       end
-      return true
+      true
     end
 
     #
@@ -102,7 +117,7 @@ module Msf
     # a valid value
     #
     def empty_required_value?(value)
-      return (required? and value.nil?)
+      required? && value.nil?
     end
 
     #
@@ -169,6 +184,4 @@ module Msf
 
     attr_writer   :required, :desc, :default # :nodoc:
   end
-
 end
-