replace plugin Smart-Tabs with vim-easy-align

The drive to make this swap was to correct the issue outlined in #57 but
overall I think this is an improvement. [EasyAlign plugin](https://github.com/junegunn/vim-easy-align/)

Reminder: you'll need to run `:BundleInstall!` to update your plugins after applying this code

Other modifications:
- Remap :A <symbol> to align on the symbol (as it did
with Smart-Tabs).
- Add a mapping for <Enter> in visual mode that triggers EasyAlign
  interactive mode for the selected lines
- Add a mapping for <leader>a, which then takes a motion/text object
  that triggers EasyAlign interactive mode for the motion/text object
  (e.g. <leader>a5j will select 5 lines and  trigger EasyAlign
  interactive mode)

Fixes #57