fix #2281: passthru escapes when parsing regex

[dgnorton]

Make the InfluxQL parser check for '/' escape and pass all other
escapes through to the Go regexp parser unmodified.

[dgnorton]

@marcosnils regular expressions go through two parsers, InfluxQL and Go's regexp. Both parsers use \ for escapes. What this PR does is make the InfluxQL parser only recognize one escape during regex parsing (\/), which is the InfluxQL regular expression delimiter. InfluxQL passes all other escape sequences through to Go's regexp parser.

[marcosnils]

@dgnorton thanks for clarifying 👍

[toddboom]

+1