[feature request] Specify location of config file

[openendings]

E.g. git pr-train --config-file=~/.pr-train-BUG-1337.yml.

Use case:

• "I am using multiple git worktrees to multitask, and want to be able to refer to a 'global' config file from all of them."
• (edit: as pointed out below, multiple trains in config solves this) "I have multiple PR trains in flight, and want to manage them all without any ln -s shenanigans."

[realyze]

Hey Chris, you can have multliple trains in the .pr-train.yml

trains:
  train for feature 1:
    - tomas/feature-1/foo
    - tomas/feature-1/bar

  train for feature 2:
    - tomas/feature-2/foofoo
    - tomas/feature-2/barbar

Does that address your issue?

[openendings]

Mostly; I would still need this when juggling multiple worktrees though. I'll amend the issue description to reflect this.