feat: themes

[rochacbruno]

Part 1:

When on config

theme: foobar

Then marmite checks if folder input_folder/foobar exists, else exits with error.
Then static and templates folder must be resolved to be input_folder/{theme}/{static_path|templates_path}

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Part 2

When running marmite folder --start-theme it will be required to pass a name --start-theme foobar

then the templates and static will be generated on foobar/

Also, the theme will be created from a very clean template NOT the examples theme.

The html itself will include documentation.