Configurability of rls-blacklist

[Xanewok]

Opened by @dweinstein (rust-dev-tools/rls-blacklist#4):

Based on reading rls-* documentation it sounded like rls blacklists are supposed to be configurable, but based on the implementation here I don't see that currently being possible. Is the goal for rls-blacklist to be configurable? via environment, config file?

[lijinpei]

There are other issues that demands a conf file or command line arguments.
#915
#1109
#1209
#1303
#1324
#1325
Maybe those issues can be solved together?

[tomusdrw]

I'd be happy to help out, but will need some code pointers and guidance, anyone available for mentoring?

[Xanewok]

@tomusdrw sorry for not responding earlier; turns out that this ended up as a yak-shave when I tried to clean up the repo a bit yesterday.
Re mentoring FWIW one can look at the changes from #1509. This spanned multiple crates so it wasn't as straightforward but I hope it makes sense. In general contributions would be more than welcome =)