Add various useful Vim plugins

A summary of all these are:

vim-abolish: three plugins in one, all related to manipulation of word similarities
vim-endwise: wisely auto-insert end after if/do/def/etc. in Ruby and other languages
vim-ragtag: key mappings for making it easy to create and edit markup languages tags
vim-repeat: expand the "." repeat functionality to also support repeating custom plugin actions
vim-surround: all about "surroundings": parenthesis, brackets, quotes, xml tags, etc.

For more information on each of these, refer to the plugin's own repo.