Add -d as short command for --max-depth

[3tilley]

I've given a go to #2643, partly just as an experiment into how easy it would be - the answer was pretty easy with the tools available.

I've not:

• Updated much documentation. The help is generated automatically, and I followed the model of --max-count / -m while making this change, where it's exclusively referred to by the long name
• Added or updated any tests - I couldn't see that the --max-depth functionality is tested anywhere, so I didn't see anywhere to add or tweak for the short form. I can add some if you would like

I have given my best go at updating the zsh completions, but I'd be lying if I said I understood exactly how it worked. I've followed the example of --max-count and --max-columns, but I'd appreciate your eye to make sure it's been implemented correctly.

I've also tested locally, and it seems to work the same as --max-depth does:

$./target/debug/rg.exe glue --max-depth 4
$./target/debug/rg.exe glue -d 4
$./target/debug/rg.exe glue -d4
$./target/debug/rg.exe glue -d=4
$./target/debug/rg.exe glue --max-depth 5
crates\searcher\src\searcher\mod.rs
20:    searcher::glue::{MultiLine, ReadByLine, SliceByLine},
27:mod glue;
$./target/debug/rg.exe glue -d 5
crates\searcher\src\searcher\mod.rs
20:    searcher::glue::{MultiLine, ReadByLine, SliceByLine},
27:mod glue;
$./target/debug/rg.exe glue -d5
crates\searcher\src\searcher\mod.rs
20:    searcher::glue::{MultiLine, ReadByLine, SliceByLine},
27:mod glue;
$./target/debug/rg.exe glue -d=5
crates\searcher\src\searcher\mod.rs
20:    searcher::glue::{MultiLine, ReadByLine, SliceByLine},
27:mod glue;

If you agree with the premise of the change, let me know if there is anything I should update or check for.

Thanks!