Feature request: expose indent_string as an optional arg to prettify()

[mmuurr]

I'd like to be able to pass \t as the indent_string to prettify() (as downstream files are diffed and the upstream -- which I can't control -- uses tabs).

Could we add indent_string as an optional argument to prettify() that:

• when present, ignores indent and
• is passed along to reformat() (perhaps after valdiating that indent_string is indeed a string)?

It's obviously somewhat easy to just use jsonlite:::reformat() instead, but I'd much rather stick with the public API, if possible.

I'm happy to gin-up this relatively simple PR if likely to be approved.

[jeroen]

Hmm maybe it is easier to overload the indent parameter than introduce a new one.

[mmuurr]

Oh, yeah ... that's a much better API/arg usage (than my indent_string idea).

[jeroen]

I think the reason why we decided it was unsafe to expose, is that yajl crashes for some values:

jsonlite:::reformat('{"x":123}', TRUE, 'x')