Get the version of the library defining FUNCTION.
If SHOW is non-nil, show the version in mini-buffer.
This function is mainly intended to find the version of a major
or minor mode, i.e.
Return the version of the library defining FUNCTION. Signal an
error if FUNCTION is not a valid function, if its defining
library was not found, or if the library had no proper version header. (fn FUNCTION &optional SHOW)