Include indent fixes

[Omnikron13]

Should fix issues #1626, #1564, #2830, #2521, #1127, #619, #1601.

Would supersede PRs #1718, #2831, #1565, and I think the majority of uses for #1856 & #2333.

There is an open question of what exactly to do with non-whitespace before a link, e.g. foo{{#include ...}}

[Omnikron13]

The relevant documentation doesn't need any changes to reflect these changes, though would need an addition if the new behaviour is considered non-obvious.

As a side note I ran into unit tests which cover undocumented behaviour while working on this. Anchored includes in the following form aren't covered atm:

// ANCHOR: name
println!("included");
// ANCHOR_END: name
println!("not included");
// ANCHOR: name
println!("included");
// ANCHOR_END: name

(As an additional detail, there were only actually unit tests for this for the rustdoc_include variant, but I have expanded the testing to cover the include version too.)

[Omnikron13]

Note: some of the issues this would fix do so relying on the fact that anything before the include link will be repeated as a prefix (because they are people wanting to use include within block quotes).