Add string method for term

[nilshg]

As discussed on Slack, this makes it easier to create Term objects from DataFrame columns, as names(df) now returns strings.

[kleinschmidt]

Looks great. Can you add something to the docs/src/formula.md file too? term is described there as well.

[kleinschmidt]

Oh and add some tests!

[nilshg]

Oh and add some tests!

Ah right I was wondering about that - I thought the jldoctest stuff is a test, so that changing the example in the docstring to include a string would be the test. Is that not actually the case or is it just that doctests are additional to normal tests, not a replacement?

[kleinschmidt]

Unfortunately not (you can see where they run in the documentation job below...rather than the travis job). We currently duplicate many of them in the actual tests. They're mostly used to make sure there aren't broken examples in the docs :)

[nilshg]

Pretty bare bones but I think this might do the trick?

[kleinschmidt]

LGTM! Last thing: bump the patch version in Project.toml? Thanks for the contribution!