Implement :substitute command

[lieryan]

What works:

• substitute current line (:s/a/b)
• substitute single line (:4s/a/b)
• substitute range (:2,4s/a/b)
• substitute from search history (/a then :s//b)
• substitute from search substitution history (:s/a/b then :s//c)
• substitute with repeat last substitution (:s)
• substitute g-flags (:s/a/b/g)

What doesn't work/not implemented:

• when substituting over a range, vim would finish by jumping the cursor to the last substitution if there is a match; this implementation always move the cursor to the last selected range
• substitution flags other than g
• substitution with count (:s/a/b/5)
• no previews (incsearch or inccommand)

[jonathanslenders]

This is amazing, thanks a lot! It's fine if not everything works. We're never going to be feature parity with Vim. But this is absolutely great.

[jonathanslenders]

I noticed a few missing things I typically use. I'm just writing them down here, just to not forget about that:

• :%s/.../.../g not yet working (should substitute in the whole document).
• making a selection and then starting substitute. (This won't bring up the command input bar for some reason. I can look into that.)