Support something simliar to context

[djmitche]

https://taskwarrior.org/docs/context.html

@srfraser mentioned in another issue that it would be handy to be able to dynamically determine contextual information from the current directory, and by running external commands. Is there a general approach we could take here?

[srfraser]

Straightening things out in my own head, there are several angles:

1. Manually setting a context as taskwarrior does, or dynamically generating it, or both
2. How taskchampion expects to receive this context - flags, tags, k:v pairs - some idea of how they'd be implemented might give clues about how to communicate them
3. If dynamic, how to determine that data
   a. Tools such as https://github.com/direnv/direnv
   b. We could try and be clever about the directory we're in and discover .git .hg directories
   c. The config file could allow a script to be set, that will be run on modification commands to generate appropriate flags

[djmitche]

I think we want to avoid baking specific interfaces like git and hg into the tool -- that opens the door to a slew of follow-on PRs to add more functionality and creeping growth. And inevitably privileges some integrations over others. Do we add a Darcs interface? SourceSafe? BitKeepr? etc.

I suspect some careful planning could generate a kind of "hook" interface that can support this. We've got time to design something nice here.

[savchenko]

There is lively conversation about contexts: #163, pending its resolution.