Closes#12.
See #499.
See #11.
This adds PyYAML as a new dependency.
It adds the following new commands:
:session-delete <name>
Delete a session.
:session-load <name>
Load a session.
:session-save [<name>]
Save a session.
:wq [<name>]
Save open pages and quit.
And the following new settings:
general -> save-session:
Whether to always save the open pages.
