Make it possible to skip code blocks based on the programming language

[mgeisler]

It feels like it would be useful to let users skip code blocks based on the info string (the part after ```).

That would make it easy to, say, skip all TOML and shell script code blocks from Comprehensive Rust.

[mgeisler]

This is an extension of #63.

[mgeisler]

@dyoo, would you also be interested in this one?

[dyoo]

Sure, I believe it should be straightforward. Yes, let me work on this.

I'm guessing we want to define a list of lang lines to ignore somewhere, perhaps another configuration option in book.toml from the proposal in #63 (comment)

[mgeisler]

I'm guessing we want to define a list of lang lines to ignore somewhere, perhaps another configuration option in book.toml from the proposal in #63 (comment)

Yeah, precisely!