Recognize theme CSS in input directory

[yhatt]

This PR will change the behavior of --input-dir (-I) option to recognize theme CSS in specified directory.

One of the expected use case is that there is theme CSS next to Markdown.

slides
├── slide1.md
├── slide2.md
├── slide3.md
├── my-theme1.css
└── my-theme2.css

Currently we have to specify folder twice in input arguments (or --input-dir option) and --theme-set option if you want to use these. It is bit redundant.

marp ./slides --theme-set ./slides

So we'll find out theme CSS from specified directory if --input-dir option was defined. Internally the directory will be added to the array of theme set option.

Thus, you would be OK just to run together with -I option in this case.

marp -I ./slides

In current version, it would throw an error and fail conversion when there is not @theme metadata in CSS found from theme set directories. We improve this behavior to output the warning which file had a problem. It no longer stop the conversion.