Global template support

[arctic-hen7]

Description
Support for a global template in ~/.bonnie/template.toml that would automatically be used instead of the pre-programmed default if it's present when bonnie -i is executed.

Reasoning
Bonnie currently lacks any form of global configuration ability. This is the best solution that provides both global configuration and atomicity and portability.

Are you willing to work on an implementation of this?
I'm happy to, but I think @ostev wanted to.

[arctic-hen7]

For reference, this was originally proposed in #12.

[ostev]

I'm currently working on a PR. I'm no Rustacean, so it will take me a little while.

One question: do you prefer ~/bonnie.toml or ~/.bonnie/template.toml?

[ostev]

Also: perhaps a merging of global with local configs would be interesting at runtime? That way you could use your own shell on open-source projects that do not use your shell.

[arctic-hen7]

Great! No problem, let me know on Gitter if I there's anything you need. I think ~/.bonnie/template.toml would make more sense.

[arctic-hen7]

Also: perhaps a merging of global with local configs would be interesting at runtime? That way you could use your own shell on open-source projects that do not use your shell.

I'm not so much in favor of this at the moment. Bonnie needs to be portable, and everyone should get the same, predictable results at runtime. Each configuration file should be entirely self-contained.