Support collapsing multi line output

[Kampfkarren]

Love the library, but there's an issue when using it with long multi-line spans--it shows every line!

I believe the lines that would need to be changed are these:

codespan/codespan-reporting/src/term/views/source_snippet/mod.rs

Lines 259 to 261 in e2d6fc3

	for line_index in ((start.line.to_usize() + 1)..end.line.to_usize())
	.map(|i| LineIndex::from(i as u32))
	{

rustc, on the other hand, will collapse multiple line spans:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=0d4c7f47b06cefb15528f7106766d5f7

Does codespan support this?

[brendanzab]

Oooh, this would definitely be a good idea! No, it's not supported yet - we'll have to add this.

[brendanzab]

I'm thinking that this will probably have to be added after #100 - I don't want to make the logic too confusing before I implement rendering overlapped labels.