Add trim option to command chains

[LucasPickering]

Add a field trim to the command chain source, which trims whitespace from the ends of the output string. It's pretty common to have shell commands that include a useless trailing newline, in which case you have to do something like command: [sh, -c, "<cmd> | tr -d '\n'"]

Worth considering: instead of just a boolean, should just be an enum? none, start, end, both?

Implementation

• Field should be added in models.rs
• Implementation in render.rs
• Documentation in chain_source.md

[Sammo98]

@LucasPickering I'd be keen to pick this one up as well, unless anyone else had plans to?