Enable doctests

[nickelc]

The doctests can be enabled by adding the necessary initialization code and hiding it with a leading #.

Doc-tests

``` Doc-tests xdg running 4 tests test src/lib.rs - BaseDirectories (line 76) - compile ... ok test src/lib.rs - BaseDirectories::with_profile (line 238) ... ok test src/lib.rs - BaseDirectories (line 49) ... ok test src/lib.rs - BaseDirectories (line 56) ... ok test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.34s ```

[whitequark]

Thanks!