Add root env vars to fix --color=always problem

[joshka]

Adds a new root level config item env, which allows setting environment
vars for all jobs. This makes it possible then to set CARGO_TERM_COLOR
to always, which fixes the problem with --color=always not working.

Fixes: #261 and #124

[Canop]

I'll test a little more today, but this looks very good.

[Canop]

Something which may seem a little disturbing: if you remove the env.CARGO_TERM_COLOR = "always" line from the bacon.toml of your project, the variable is still set, because the default bacon.toml is used to init settings and env vars are only extended.

This doesn't look like it can be a problem, it's even good as it will make it possible for users to add new jobs without the --color always thing even if they don't have the env.color = always line at the top.

But the fact that you can't, today, remove an env var defined previously (in the prefs.toml or in the bacon.toml root) may be seen as a problem. Maybe we'll have to add support for an ugly inherit_env = false in the future ?

Right now, I think what you did is the way to go.

[joshka]

Yeah, I was thinking about that as a potential problem too. Perhaps making the hashmap accept Option<String> would work for unsetting vars.

I also didn't add env to the args, which might be nice too, mainly because I wasn't sure the syntax for representing --env FOO=bar in clap (and didn't find an obvious answer in a few minutes of search)