Add '!include' statement

[ghost]

Replaces #53. Could use a check on the path resolution logic to see if it's correct.

[emersion]

Right now profile names make this ambiguous (a profile could be called !include). Potential solutions:

• Disallow profile names starting with !
• Add a mandatory profile prefix: profile my-profile { … }
• Make quotes mandatory for profile names: "my-profile" { … }

[emersion]

Add a mandatory profile prefix: profile my-profile { … }

I think I'm going to go with this solution. See https://github.com/emersion/kanshi/issues/75

[emersion]

We should now be able to use the top-level include directive safely.

[emersion]

Superseded by #87