add Quote type to enable safe concise output of untrusted strings

[schmichael]

Output from hashicorp/nomad#10858 using hclog v0.14.1 (double double quotes):

2021-07-06T10:06:01.590-0700 [WARN] ... task=redis health=critical output=""This is the output for state 2\n""

Output using hclog v0.16.1 (breaks naive 1-line-per-log parsers):

2021-07-06T15:41:34.112-0700 [WARN] ... task=redis health=warning
  output=
  | This is the output for state 1

Output using this PR:

2021-07-06T15:42:28.192-0700 [WARN] ... task=redis health=warning output="This is the output for state 1\n"

[schmichael]

Left open for a while to consider other names. Almost went with Escape like in Javascript, but Javascript's escape uses %NN notation instead of \x notation like Go's strconv.Quote and JSON. Keeping the Quote name and merging.