Add tags completion support

[zhaocai]

Refer to #71

To support custom filetype like autohotkey, the LANG_TO_FILETYPE map inside ycmd\cpp\ycm\IdentifierUtils.cpp
needs to be patched.

// List of languages Exuberant Ctags supports:
//   ctags --list-languages
// To map a language name to a filetype, see this file:
//   :e $VIMRUNTIME/filetype.vim
// This is a map of const char* and not std::string to prevent issues with
// static initialization.
const boost::unordered_map < const char *,
      const char *,
      boost::hash< std::string >,
      StringEqualityComparer > LANG_TO_FILETYPE =
        boost::assign::map_list_of
        ( "Ant"        , "ant"        )
        ( "Asm"        , "asm"        )
        ( "Awk"        , "awk"        )
        ( "autohotkey" , "autohotkey" )