Allow theme and lexer for RST and Markdown printing

[wasi-master]

As it stands currently, rich-cli prints the rst/markdown document without considering the lexer or theme argument

Markdown: https://github.com/Textualize/rich-cli/blob/main/src/rich_cli/__main__.py#L564 RST: https://github.com/Textualize/rich-cli/blob/main/src/rich_cli/__main__.py#L570

What I suggest is to get the --theme argument and pass it to the code_theme keyword argument of the Markdown and RestructuredText class, and optionally the --lexer argument to the default_lexer parameter of the RestructuredText class

I may open a PR later if @willmcgugan is intrested

[willmcgugan]

Makes sense! 👍

[wasi-master]

PR coming :)