Indent guide

[rhysd]

Consider to add indent guide to the result snippets. Since snippets usually include indents and sometimes the indent level is deep, it would be helpful to know the level.

    ┊   match self.regions.first().copied() {
    ┊   ┊   Some((s, e)) if o == s && o < e => RegionBoundary::Start,
    ┊   ┊   Some((_, e)) if o == e => {
    ┊   ┊   ┊   // When the next region is adjcent, skip changing highlight
    ┊   ┊   ┊   match self.regions.get(1) {
    ┊   ┊   ┊   ┊   Some((s, _)) if o == *s => RegionBoundary::NotFound,
    ┊   ┊   ┊   ┊   _ => RegionBoundary::End,
    ┊   ┊   ┊   }
    ┊   ┊   }
    ┊   ┊   _ => RegionBoundary::NotFound,
    ┊   }

• Character: ┊ (U+250A)
• Color: gutter
• Option: --no-indent-guide (indent guide is enabled by default)

Note that this feature is automatically disabled by --ascii-lines.

Challenges:

• How to calculate indent width?
  • Width would be calculated at making chunks from grep matches.
• How to handle tab character?
• How to propagate the calculated indent width to a printer?

I should to confirm that the guides are not so noisy in outputs before implementing this by crafting an example output.