Add variable to autoload sessions

[evanthegrayt]

Closes #3

Adds variable to autoload session on VimEnter and DirChanged
Remove the ability to specify a session

[evanthegrayt]

This is working... but for some reason, when the session is automatically loaded, I lose my statusline color.
How it looks

How it should look

[evanthegrayt]

I've narrowed the issue down to set background=dark in the session file. Maybe a startup race condition? Having difficulty troubleshooting this one.

[evanthegrayt]

Hmm, it's because Airline loads before. Adding AirlineRefresh to my autocmd fixes it... but I don't want to add a bunch of conditionals for different status lines for something so simple. Unsure how to proceed -- I don't like releasing this with a known issue.