Don't cache CtrlP results when using ag

It gets very annoying to manually force CtrlP to re-index every time I
add a new file. Since ag is so fast, we can turn off caching when it
powers CtrlP without any noticeable impact on search speed.
Allow backspacing over everything in insert mode

The previous setting frequently slowed my ability to edit.
Set 'autowrite'

* Automatically :write before commands such as `:next` or `:!`
* Saves keystrokes by eliminating writes before running tests, etc
* See `:help 'autowrite'` for more information
Add envup script for loading environment variables