Code snippets: Support additional filtering when displaying

First, thanks a ton for presenterm, I think it's a great presentation tool! I am currently preparing a presentation including a lot of code snippets directly from a java project. As it is, these contain a lot of `import` statements at the top, that I don't necessarily want to show in my slides. I could use the `hidden_line_prefix` to hide those, but then the code would not compile anymore in my java project. Have you considered using something like `hidden_line_regex`, that in my example would contain `^import .*` to exclude this lines at the top of a file, but should include the rest, without changing the structure of the file, so it can still be used in the Java project. Does it even make sense to you? :-)