feat(expert): make log level for log files configurable

[katafrakt]

This adds a command-line flag --log-level which affects the filtering of the log level in expert.log and project.log. It keeps :debug as default.

Addresses #541 (I don't want to close it automatically, as we may want to continue there for feedback)

First few logs on info level are kept always for debugging. This is when I set the log level to error:

17:07:10.045 instance_id=19D6DA20D51 [info] Expert v0.1.0 starting on stdio, argv=["--stdio", "--log-level", "error"]
17:07:10.046 instance_id=19D6DA20D51 [info] Log level set to error

[katafrakt]

Currently this approach has too many problems to be ready for review. It affects editor log level too, among others.

[emdvtc]

When reporting the verbose logging issue I've tried my hand at setting the log level and compiling the app. I found logging quite complicated, how come you don't use the standard Logger ?

[doorgan]

@emdvtc we did until we unified elixir and LSP logs
we're not doing anything wild outside of that, just forwarding them to .log files, from both the main manager node and the engine nodes

[katafrakt]

I like #563 more, I think this should be closed, because setting CLI flag is not very user-friendly.