Custom conversion doesn't work about toml11

[ToruNiina]

Thank you @htrefil for the report!
And thank you @jcmoyer for the PR and finding out the problem!

This is a bug.
As you pointed out, there is a defect in the code and it requires conversion in both directions even if only one direction is needed. I've just fixed that and will push it soon.

[jcmoyer]

I think the documentation is incorrect (from_toml should be static), see this test for correct usage: https://github.com/ToruNiina/toml11/blob/master/tests/test_extended_conversions.cpp#L38-L57

EDIT:
After a bit of testing, I've determined that there's another issue where you have to specialize both toml::into and toml::from for your type, even if you're only reading toml. I'm not sure if this is intended or a bug.