Add `invert` arg to `stri_locate_*`

```

invert_match {stringr} R Documentation
Switch location of matches to location of non-matches.

Description

Invert a matrix of match locations to match the opposite of what was previously matched.

Usage

invert_match(loc)
Arguments

loc
matrix of match locations, as from str_locate_all

Value

numeric match giving locations of non-matches
```

cf. https://github.com/hadley/stringr/issues/50