Use custom variable to track session loading state

[tmandry]

mkview creates a script which calls doautoall SessionLoadPost without setting g:SessionLoad. When run with loadview, this can cause problems because we are loading and saving to a session at the same time.

Specifically, when running with a NERDTree window, I saw buffers being copied to that window, and some buffers disappearing from other windows.

I still need to check for g:SessionLoad, though, when loading a session with xolox/vim-session. Otherwise only one buffer per window gets loaded. Not sure why that is.

Fixes #59