Make it easier to add or remove multiple packages w/ Vector{String}

[mkitti]

Currently one cannot add or remove packages as indicated by a Vector{String} see #134

This pull request allows for the following syntax.

CondaPkg.add(["jsonlines", "cowpy"])
CondaPkg.rm(["jsonlines", "cowpy"])

It implements this by creating a function _to_spec with two methods.
For an existing *Spec, it just returns the spec. For an AbstractString,
it returns a PkgSpec.

[cjdoris]

@mkitti Thanks! Looks good. However the tests are failing on Windows for some mysterious reason that seems irrelevant to this PR (#137), I'm assuming it's a bug in micromamba. Can you replace the 4 packages in the test with jsonlines and cowpy since these are smaller and faster to install and aren't numpy please? This should make the tests pass.

[cjdoris]

Thanks 🙂