StringIndex idea (Julia 2.0)

[mnemnion]

@mnemnion

But a syntax such as string[1ch:5ch], string[1gr:5gr], string[1wc:5wc] is a nicely-compact way to generate those sorts of index. wc because of the wcwidth function which never quite made it into the C standard, it could be 1tw to remind users that it’s using the textwidth function under the hood. I think this is fitting because the various approaches to indexing are effectively units, and this looks like a unit, and units are a place where highly-abbreviated names are considered acceptable, no one insists on 1m being spelled 1meter.

So I went ahead and implemented this.