Fast current_indent implementation

[Danielkonge]

This is the most up to date implementation mentioned in #649, and I don't feel any slowdown compared to the version without current_indent.

Please let me know if you want me to change something, then I can edit it during the next few days.

[Danielkonge]

Even if you end up not accepting this pull request, I think it is worth considering changing utils similar to what I have done with start_pos_scope and end_pos_scope in utils.lua. It worked more consistently for me at least. (Even without the old definitions too, I just left them to have more chances of hitting the color you want.)

[lvim-tech]

@Danielkonge thanks for your work. I think indentation doesn't work correctly in all cases

[Danielkonge]

I found a bug where current_indent wasn't correct if you jumped back more than one indent level in a single line. I have fixed it now. I think that is what affected you too (going two levels back from line 370 to line 371). Could you let me know if the problem is fixed in the newest commit?

[lvim-tech]

@Danielkonge now work correctly!

[Danielkonge]

By now this commit has actually got a few different features mixed into it.

It has current_indent as stated in the title of this pull request, but since this is really just my working implementation of indent-blankline, more stuff has gotten included.

It now also adds a show_exact_scope and a current_block_only option + some improvements to the highlight hook for the scope.

Would you prefer that I split these out into multiple smaller pull requests @lukas-reineke? Or is it easier for you to do it all at once in one bigger file (like now)?

[lukas-reineke]

Please make smaller PRs 👍
And it would be nice if you can write some tests for it too

[Danielkonge]

@lukas-reineke I have submitted 3 smaller pull requests now, and I will add another smaller one with only current_indent later (I will leave this one open until that one is ready).

I am not sure how exactly you want to test these features, but if you clarify what kind of tests you want, I can try to write some tests too. [I didn't see any tests for scope to base my tests on.]

[Danielkonge]

I finished the new smaller and cleaner pull requests now.