Postgres plugin: option to remove password in tag

[donaldguy]

Adds option to avoid server tag including password. Disabled by default
for backwards compatibility

This version uses pq.ParseURL to canonicalize to key-value format
before doing sanitazation. So inputs in URL format that enable this option will
switch formats. It easy enough to make a version that removes password while
preserving URL format by adding branching logic with net/url.

It is unclear to me which is preferable. Canonicalization might be seen as a virtue. Also, its tempting to take it one step forward and make host, user, etc. seperate tags rather than a concatted string.

I am open to any suggestions as to the name of the config_key, etc.

Thanks 😄

[sparrc]

Thanks @donaldguy, please try to rebase this change off of the 0.3.0 branch and have the PR merge to that branch as well, thank you!

If you would like to break backwards-compatability on the 0.3.0 branch, it's no problem, just make a note of it in CHANGELOG.md

thanks again!

[donaldguy]

Closing in favor of #490