Accept dotted-paths of the form path.to.module:object1.object2

[pawamoy]

Is your feature request related to a problem? Please describe.
When importing an object with only dots in its path, it is hard to know what is a module and what is an object. It leads to confusing tracebacks: #53, mkdocstrings/mkdocstrings#127

Describe the solution you'd like
To ease the process of importing the object, and to be able to return better error messages, we could support having a colon in the object's path. This colon would delimit the last submodule from the first object. We wouldn't need to try and import the whole path, going up one level each time, to find the deepest submodule. An import error happening in the deepest submodule would be reraised as is, clearly showing what the problem is to the user in mkdocstrings logs.

Describe alternatives you've considered
Additional checks as proposed in this comment.

[pawamoy]

Implemented in a0b677c, published in version 0.9.0.