attribute to require interactive terminal

I've made a PoC for configurations attributes: `[interactive]` and `[not-interactive]` which can enable/disable recipes based on the equivalent of `[ -t 1 ]` in bash.

I've come across a sitation where I want to run slightly different commands in either situation but switching the recipe to use bash makes it kinda ugly. This may also help in a situation where an agent is selecting a recipe to not getting stuck in a TUI, for example.

Here's the diff: https://github.com/casey/just/compare/master...robjtede:just:interactive-attribute

Not sure what you think about adding this as a configuation attribute, given the only others of those are the OS selection ones. Didn't take much time to write up so if you're opposed in general no harm done. Otherwise, I can get that diff in a better state and submit a PR.