Implement config

[barrettj12]

Config will be stored at the following levels:

• global config - stored in $HOME/.jit/config.yaml
• repo-specific config - stored in <repo-root>/.jit/config.yaml
• (branch specific config?)

Config values are decided in the following precedence order:

• command flags/args
• environment variables
• (branch specific config?)
• repo-specific config
• global config
• hardcoded defaults

Config is stored as plain string maps in a yaml file. Given a config file like

sec:
  map:
    key: value

the full key for value is sec.map.key.

A jit config command will be added to set and get config values. Get a config value with

jit config key

Set repo-specific config with

jit config key=value

Set global config with

jit config --global key=value

[barrettj12]

Might be over-designing it a bit. We can just make everything a top-level key for now and see how it evolves.