Add --images option to support conversion into multiple image files

[yhatt]

Resolves #71. Added --images option to support conversion into multiple image files by each slide page, like output.001.png, output.002.png... Usage is same as --image option.

$ marp markdown.md --images png
[  INFO ] Converting 1 markdown...
[  INFO ] markdown.md => markdown.001.png
[  INFO ] markdown.md => markdown.002.png
[  INFO ] markdown.md => markdown.003.png

I've refactored internal logic of Converter class to support multiple image conversion for every page. It would be taken screenshot multi-times from a page without extra loading.

--image option for title slide page is still supporting to keep compatibility, but we may consider a new way taking the specified page(s) with --images option. If we added new option(s), --image would obsolete in favor of using --images and page option.

[yhatt]

Examples (using yhatt/marp-cli-example):

$ npx marp PITCHME.md --allow-local-files --images png