Fix projectfluent#7. Introduce tags

Tags are binary values attached to messages.  They are language-specific and
can be used to describe grammatical characteristics of the message.

    brand-name = Firefox
        #masculine

    brand-name = Aurora
        #feminine
        #vowel

Tags can be used in select expressions by matching a hashtag name to the
message:

    has-updated = { brand-name ->
        [masculine] …
        [feminine] …
       *[other] …
    }

Tags can only be defined on messages which have a value and don't have any
attributes.

Author: stasm

syntax/fluent.asdl

@@ -23,7 +23,7 @@ module Fluent
 {
     res = Resource(entry* body, comment? comment)
 
-    entry = Message(iden id, pat? value, attr* attributes, comment? comment)
+    entry = Message(iden id, pat? value, attr* attributes, tag* tags, comment? comment)
           | Section(symb name, comment? comment)
           | Comment(comment)
           | Junk(string content)
@@ -45,6 +45,9 @@ module Fluent
     -- Attributes of Message
     attr = Attribute(iden id, pat value)
 
+    -- Tags of Message
+    tag = Tag(symb name)
+
     -- Variants of SelectExpression
     var = Variant(varkey key, pat value, bool default)
     varkey = Number(number)

syntax/fluent.ebnf

@@ -34,10 +34,13 @@ variant              ::= NL __ '[' _? variant-key _? ']' __ pattern
 default-variant      ::= NL __ '*[' _? variant-key _? ']' __ pattern
 variant-list         ::= variant* default-variant variant*
 
+tag                  ::= '#' word
+tag-list             ::= NL (__ tag)+
+
 attribute            ::= NL __ '.' identifier value
 attribute-list       ::= attribute+
 
-message              ::= identifier (value attribute-list? | attribute-list)
+message              ::= identifier ((value tag-list?) | (value? attribute-list))
 value                ::= _? '=' __? pattern
 pattern              ::= (text | placeable)+
 text                 ::= ((char - line-break) - special | break-indent | '\' special)+