Lektor 613 - Passing extra_flags to all plugin events

[nixjdm]

Fixes #613.

This passes extra_flags to every plugin event, and allows extra_flags in a few more commands like clean and dev shell. This includes tests for most of the added functionality.

The tests could be improved, but the bigger deficiencies are not easy to overcome:

• Testing the events hooked by lektor server; server_spawn and server_stop. Maybe there's an easy way I haven't thought of?
• Including cli triggered hooking of the markdown config commands. Tough because of the markdown thread local.
• Cleaning up after creating and installing plugins within the single pytest command. That's much more difficult than I originally realized. See inline comment block.

IMO, none of those are bad enough to block this. I'll happily accept recommended changes though if someone thinks of something I haven't :)

Ping @Andrew-Shay, for lektor/lektor-website#256.

[skorokithakis]

This looks good to me at a first glance, but I'm not super duper familiar with the codebase.

[nixjdm]

Looks like I messed up the working directory pathing of the tests a bit. I'm fixing that.

Thanks for the look!

[nixjdm]

I think I'm done modifying this, tweaking the tests.

[goanpeca]

Looks pretty good in general :-)