Accept capital strings in Level.UnmarshalText

[jcorbin]

Makes it easier to parse zap's own log output (especially since capital level strings is our internal standard).

[prashantv]

Should we just strings.ToLower(string(text)) so that we accept debug Debug and DEBUG?

[jcorbin]

• I'd prefer to avoid introducing an alloc as I'm using this on the hot path (external down sampler) and not just as part of startup config parse
• while I understand the impulse to "Parse Them All! ™", the all-caps version actually is more privileged since it's one of the configurable formats that zap itself will produce
• my real intention with this pr was: "if we can write it (and we do by default internally), we should parse it"

[akshayjshah]

@jcorbin What do you think of pulling out the switch into a separate function and making two attempts at parsing - once with the original string, and once with the lowercased version if the first pass fails? That keeps the success case fast, but successfully parses a wider array of reasonably well-formed inputs.

[akshayjshah]

Excellent.