caddyfile: Support backticks as quotes

[mholt]

Slated to land just after 2.0 release.

Closes #2591

[francislavoie]

Nice and simple! 👍

[Conan-Kudo]

@mholt Why use backticks instead of """? That would align similarly with other languages that do this (like Python, Ruby, et al)?

[francislavoie]

Since the primary usecase for this is JSON strings, backticks make more sense here I think. Easier to read, and JS has support for backticks as "template literals", i.e. interpolated strings, and Go does as well, "string literals".

Compare:

respond `{"some": "JSON", "content": ["1", "2", "3"]}`

vs...

respond """{"some": "JSON", "content": ["1", "2", "3"]}"""

And it's easier to implement in the lexer because otherwise we'd need to special case finding three consecutive ". Not that big of a difference, but simpler is better.

[elcore]

LGTM 👍