Support escaping dots in CSS identifiers

Escaping hash characters and whitespace is _not_ supported as neither
are valid element/attribute names (e.g. <foo#bar /> is invalid
XML/HTML).

Escaping single/double quotes also won't be supported for the time
being. It's quite a pain to get this to work right in not just CSS but
also XPath and XML/HTML, for very little gain. Should there be enough
users with an actual use case (other than "But the spec says ...!") I'll
look into this again.

Fixes #124

lib/oga/css/lexer.rl

@@ -49,6 +49,7 @@ module Oga
       # @see [#add_token]
       def advance(&block)
         @block   = block
+        @escaped = false
 
         data  = @data # saves ivar lookups while lexing.
         ts    = nil
@@ -143,10 +144,22 @@ module Oga
         # Identifiers are used for element and attribute names. Identifiers have
         # to start with a letter.
 
-        identifier = '*' | [a-zA-Z_]+ [a-zA-Z\-_0-9]*;
+        ident_word = [a-zA-Z\-_0-9]*;
+
+        ident_escape = '\\.' %{ @escaped = true };
+
+        identifier = '*' | [a-zA-Z_]+ ident_word (ident_escape ident_word)*;
 
         action emit_identifier {
-          emit(:T_IDENT, ts, te)
+          value = slice_input(ts, te)
+
+          # Translates "foo\.bar" into "foo.bar"
+          if @escaped
+            value    = value.gsub('\.', '.')
+            @escaped = false
+          end
+
+          add_token(:T_IDENT, value)
         }
 
         # Operators

spec/oga/css/lexer/paths_spec.rb

@@ -10,6 +10,18 @@ describe Oga::CSS::Lexer do
       lex_css('_h3').should == [[:T_IDENT, '_h3']]
     end
 
+    it 'lexes a path with an escaped identifier' do
+      lex_css('foo\.bar\.baz').should == [[:T_IDENT, 'foo.bar.baz']]
+    end
+
+    it 'lexes a path with an escaped identifier followed by another identifier' do
+      lex_css('foo\.bar baz').should == [
+        [:T_IDENT, 'foo.bar'],
+        [:T_SPACE, nil],
+        [:T_IDENT, 'baz']
+      ]
+    end
+
     it 'lexes a path with two members' do
       lex_css('div h3').should == [
         [:T_IDENT, 'div'],