Save scrollback and start logging as single function

[bedge]

Fixes #31

Note that a cleaner solution may be to restructure the toggle/start scripts so that the block I added didn't need to replicate the context read that already exists in the toggle script.

[bedge]

Updated PR to fix #38 as well

[bedge]

OK, got carried away. Added README updates, and key bindings for the snapshot_and_log.
Hope this fits with your plans.

[bedge]

Can I get a review on this?
Been working splendidly for me:

Enabling shows this on the statusline:

History saved and logging started to /Users/bedge/log/tmux/2019-08-13/deng-2.log

Lets you "forget" to enable logging for an important thing, then go back and log the whole scrollback buffer and start logging everything after that.

[codydiehl]

Is there a way to stop the logging once it has began using the snapshot_and_log.sh ?
seems the toggle_logging.sh simply starts a new logging session when this is already logging the current pane.

I think it might be good to integrate an additional stop function. While not a necessity it would be full featured to do so.

Looks like would need to track whether we are already logging with a similar yet new function to the is_logging() found in the toggle_logging.sh script and then the function to turn it off by simply running the script again in the same pane that it was already started within.

I will take a look if i can get some time, but wanted to post here to see if there was something I was missing before trying to implement it myself.

[bedge]

@codydiehl, I agree completely.
This PR was a quick hack to add a missing toggle. I expect there's more than one case it's missing.
Glad to see it's getting a second look.