String functions: hasPrefix, hasSuffix, etc

[aclowkey]

How about adding some string functions such as hasPrefix, hasSuffix, regexp.Match?
EDIT: Oh I see there's regexp.Match() which should cover all other cases.
But maybe it's still worth to add these simple ones as most programming languages have them without regexes..?

[blake]

Support for sprig functions was added in #1312 and made available in consul-template version 0.28.0.

The Sprig library has hasPrefix and hasSuffix functions that can be used within Consul template by prefixing the function name with sprig_. E.g., sprig_hasPrefix and sprig_hasSuffix.