--color always revisited

[joshka]

Cargo has a CARGO_TERM_COLOR environment variable which can be used to force colored output.
Adding a new top level item that allows environment variables to be set for all commands seems like it might solve #124 properly without having to litter commands with --color always in the config.

I'd suggest expanding #146 to make global environment vars.

The workaround of setting these in the .cargo/config.toml is not quite right as that would affect all cargo usages, but I only want to affect usages of cargo from Bacon.

[Canop]

This is very interesting, I didn't know this variable.

[joshka]

This is very interesting, I didn't know this variable.

Neither did I until yesterday :)