Add support for asciidoc files

[lmihalkovic]

@mbanzi stated in the dev mailing list that asciidoc (.adoc) will be the preferred file format for Arduino Create to provide readme information (It seems some legacy libraries are using markdown, probably owing to their being hosted on GH) in sketches. It would be nice if IDE could also display/edit these files

[lmihalkovic]

it would probably make sense to share the CSS between the 2 environments for more consistency

syntax coloring is deplorable for the moment...

[lmihalkovic]

Due to lack of interest