Configure code_prettify extension to load custom yapf styles by default.

[artificialsoph]

Use case: I use a custom yapf style in my projects and I would like to configure the code_prettify extension to load that custom style.

Currently, this can be done by modifying this extension's configuration, which works fine for individual modification. However, I think more sensible behavior is for the code_prettify extension to by default follow the same behavior described in yapf

YAPF will search for the formatting style in the following manner:

1. Specified on the command line
2. In the [style] section of a .style.yapf file in either the current directory or one of its parent directories.
3. In the [yapf] section of a setup.cfg file in either the current directory or one of its parent directories.
4. In the ~/.config/yapf/style file in your home directory.

If none of those files are found, the default style is used (PEP8).

method: I accomplished this by using yapf's file_resources submodule, which should help ensure that this extension keeps up with any potential future changes from the yapf team.

testing: I've tested out this configuration using the contributing guidelines.

[juhasch]

I don't know much about yapf,but it looks good to me. Could you add a short description on how to load a custom style in the readme ?

[artificialsoph]

@juhasch Thanks! Done 😄