Screen-brightness-control

* Fixed issues 7 and 8
* Made lower-level windows functions more consistant with their output types

* Patched issue 6

* Merged commit from drojf in issue 5, fixing `fade_brightness`

* Improved the way monitor manufacturers are processed
* Fixed the CLI not working with single monitors
* Fixed CLI not working with xbacklight
* Fixed some inaccurate docstrings
* If a monitor's brightness method fails `None` is added to the output instead of nothing. Output length should now always match length of `list_monitors()`

* Removed `screen_brightness_control.windows.WMI.get_display_serials` as it wasn't useful
* Re-wrote most docstrings to be able to provide full documentation for the project, which is now hosted on https://crozzers.github.io/screen_brightness_control. I will add to it as I go

* Added `method` kwarg to brightness methods so that you can choose how the brightness is adjusted. On windows the options are `'wmi'` and `'vcp'` and on linux they are `'light'`, '`xrandr'` and `'xbacklight'`
* Added method command line arg
* Merged pull request from `lcharles` 2 to add top-level function `list_monitors` (Windows only), add a command line option for this and add a function in the windows module called `list_monitors_with_method`, which returns list of pairs of display names and their methods
* Fixed the `XRandr.get_brightness` function returning the wrong type