conda support: refactor check_conda & helpers, conda-related config fields

I was originally thinking implementing smuggling via conda would work similar to how it does for `pip`, where we could simply replace the `conda` executable that'd normally be looked up in `$PATH` with the full path to the given environment's exe. But looking at different conda versions and distributions, there usually isn't a separate `conda` executable, which means we'll need to implement this more like how IPython does so, by calling the base environment's `conda` executable and passing the particular environment's `--prefix` when appropriate.

Looking at what we'll want to lazily evaluate, store, make editable vs read-only, etc.), it quickly became clear that the most sensible implementation of this would require a good bit of refactoring across a number of modules. So for the initial release I'm leaving this as-is and just patching `davos.implementations.ipython_common` to fix #53, since conda isn't supported by v0.1.0 anyway. When we eventually want to support conda, I'll circle back to this more serious refactor.