use 'i' instead of 'e' for empty buffer

I guess most people are used to just write away via 'i' after entering
Vim. So this change adheres to them. (Say "Thanks!" to @Osse)

If you don't like this change consider creating an additional, hidden
key for creating the empty buffer:

let g:startify_empty_buffer_key = 'e'
change startup check regex once again

I'm terribly sorry for excluding all MacVim users.

Closes #14.