Project file to persist projects

[ThomasFeher]

Currently the user has to remember (or rely on command history) the exact
parameters for the :BobProject command. It would be nice, if vim-bob would
create a kind of project file that stores these information.

Two new commands :BobSave and :BobLoad seem appropriate for that.

Automatic saving after successful :BobProject seems also sensible.
That seems to be the more important part (together with :BobLoad, of course),
as it would probably cover most of my use-cases.
:BobLoad without arguments could just load the automatically saved last
project.