Vim: add virtualenv segment

Fixes #680
VimPowerline: catch exceptions in `render` and return a str/error
Vim: add window_title function, used in quickfix theme

This is used to display the window title of the quickfix window. Both
Syntastic and Vim use this. See w:quickfix_title.
Merge branch 'vim-window-title' into develop

Conflicts:
	powerline/segments/vim.py
Merge branch 'vim-virtualenv' into develop
Merge branch 'handle-exceptions-configloader-load' into develop