get endpoint idea

[jvanasco]

fixes #377

adds /get endpoint, which requires the same params as /update BUT does not write the txt value and returns the existing txt value

Originally: I would rather have not required a subdomain for POST, but this leverages the existing auth AND works as a decent dry-run . someone more familiar with Go could write a much better PR. this is just a proof of concept.

Now: Requiring the subdomain makes sense, and I refactored the code.

[Tragen]

Shouldn't this be api.GET instead of api.POST as with api.GET("/health", healthCheck)?
POST is for changing data.
Sure it will work but it looks strange to me.

[jvanasco]

This is an authenticated call, so should be sent in POST. /health is not authenticated.

[coveralls]

coverage: 73.291% (+0.02%) from 73.276%
when pulling 8594bf4 on jvanasco:feature-check
into b7a0a8a on joohoi:master.

[jvanasco]

Added tests and docs.