pillar support

[krlmlr]

• utf8_encode(quote = NA), meaning "quotes if needed"
• enhance utf8_encode(escapes = ...) to support subtle quotes, perhaps passage of full open-close strings, and perhaps fix utf8_encode(escapes = "31") colors escapes and backslashes/quotes differently #34 by offering an alternative option
• Understand difference between utf8_encode() and utf8_format(); only the latter has a chars argument (but utf8_format(quote = TRUE) broken? #33?), only the former has escapes
• When abbreviating, never break escapes and honor wide characters

For r-lib/pillar#563.

CC @patperry.

[patperry]

I don't remember the exact details but encode and format are meant to follow base R (encodeString and format). The former is for encoding control characters. The latter is for formatting a string immediately prior to printing.