Make options more consistent

[astrofrog]

Some options are available only through the decorator (e.g. the style), while some are accessible as command-line options and one (the results dir) is also an ini option. We should make this more consistent so that e.g. styles can be set globally on the command-line or in an ini file.

[astrofrog]

@dopplershift - could you explain why you added mpl-results-path as an ini option in addition to a command-line option? I started adding other options as ini options but then noticed that most other plugins don't really do this, so I was wondering whether we even need ini options at all?

[dopplershift]

I like the ini options because it lets me put our standard workflow in setup.cfg, and then running our tests is just pytest or python setup.py test.

[astrofrog]

You don't need the ini option to use setup.cfg though since you could also add any option to the [pytest] section in setup.cfg anyway?

[dopplershift]

I have mpl-results-path listed under [tool:pytest] currently. I was under the impression that this didn't work until I added the ini section support for it. Am I mistaken?

[astrofrog]

@dopplershift - I have to double check, but I know for sure you can do:

[tool:pytest]
addopts = --mpl

without the ini.

[dopplershift]

I didn't know about addopts. That seems to work fine for my cases, so I'm ok with removing the direct ini options.

[dopplershift]

Ok, here's one reservation I have after working with addopts. The ini option specific to pytest-mpl is ignored if pytest-mpl isn't installed, whereas the addopts is always on. This produces errors if pytest-mpl isn't installed.