doctest for modules without documentation and for separate functions

[xvries]

It would be nice to be able to run doctest() on:

1. A module without a */docs/src/ directory. Right now it throws an error.
2. A separate function, in order to troubleshoot a single docstring.

[mortenpi]

For (1), you should be able to do it with doctest(MyModule, manual = false). (2) would indeed be nice to have.

[odow]

Closing a duplicate of https://github.com/JuliaDocs/Documenter.jl/issues/1213