Document trybuild cfg option

When running tests under `trybuild`, it may be useful to disable some parts, especially if they depend on a specific compilation environment that `trybuild` does not provide. Looking through the source, I can see that the rustflag `--cfg trybuild` is set, which would allow for such functionality of a macro. This cfg option is not documented anywhere in the README. I would add it to the readme, but I'm not too sure how to document such a thing.

This change could also remove the need for #105 since the macro would be able to detect it is running in an std environment and not have to deal with the `eh_personality` and `panic=abort` mess. It would be nice to be able to build/run trybuild tests in a no_std environment, but the standard testing tools run tests in an std environment anyways, so this would stay consistent.