Fixes #439

Problem

With a numpy-style docstring, an argument whose name happens to match a section title keyword (argument, arg, param, parameter, key, ...) breaks help text alignment:

def run(executable, argument):
    """Run.

    Parameters
    ----------
    executable :
            Executable to run
    argument :
            Positional argument to the executable

    """

--help previously showed both descriptions under EXECUTABLE and nothing under ARGUMENT:

POSITIONAL ARGUMENTS
    EXECUTABLE
        Executable to run

        Positional argument to the executable
    ARGUMENT

Cause

Inside the numpy Parameters section, the line argument : sits at the same indentation as the section header, so _google_section_permitted allows it to start a new section, and _google_section matches argument against the Args section titles. The parser switches to a Google-style Args section, so the argument is never recorded and the following description line is appended to the previous argument.

Fix

Numpy parameter lines have whitespace before the colon (argument : int), while Google section headers do not (Args:). When the parser is inside a numpy-format section, a line whose colon is preceded by whitespace is now treated as a parameter definition rather than a new section header.

With the fix, --help shows:

POSITIONAL ARGUMENTS
    EXECUTABLE
        Executable to run
    ARGUMENT
        Positional argument to the executable

Added a regression test using the example from the issue. All existing docstring and helptext tests pass.