The payload-hash-* options are imprecisely named. Clarify their function
as HMAC validations by renaming them. The existing options will continue
to work but are deprecated. Log a warning if the old options are used.
All tests, examples, and documentation are updated.
Fixes#289
Add a -template command line option that instructs webhook to parse the
hooks files as Go text templates.
Includes a `getenv` template func for retrieving environment variables.