ENH: Allow to specify different ctags tool for particular languages.

Implement this via a new configuration option g:easytags_languages, a Dictionary that contains the ctags languages as keys, and corresponding special cmds, args, etc. as values.
Merge branch 'enh-languages' of git://github.com/inkarkat/vim-easytags into inkarkat-enh-languages
Move g:easytags_languages documentation to README.md

Integration of pull request #40:
https://github.com/xolox/vim-easytags/pull/40