in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

closes #1542

Generalize event.

Add doc.

Update default config.

Add filestack to the list of plugins.

Check that video conversion event returns 400.

Update the readme.

Update the changelog.