* plugin/sign: a plugin that signs zones

Sign is a plugin that signs zone data (on disk). The README.md details
what exactly happens to should be accurate related to the code.

Signs are signed with a CSK, resigning and first time signing is all
handled by *sign* plugin.

Logging with a test zone looks something like this:

~~~ txt
[INFO] plugin/sign: Signing "miek.nl." because open plugin/sign/testdata/db.miek.nl.signed: no such file or directory
[INFO] plugin/sign: Signed "miek.nl." with key tags "59725" in 11.670985ms, saved in "plugin/sign/testdata/db.miek.nl.signed". Next: 2019-07-20T15:49:06.560Z
[INFO] plugin/file: Successfully reloaded zone "miek.nl." in "plugin/sign/testdata/db.miek.nl.signed" with serial 1563636548
[INFO] plugin/sign: Signing "miek.nl." because resign was: 10m0s ago
[INFO] plugin/sign: Signed "miek.nl." with key tags "59725" in 2.055895ms, saved in "plugin/sign/testdata/db.miek.nl.signed". Next: 2019-07-20T16:09:06.560Z
[INFO] plugin/file: Successfully reloaded zone "miek.nl." in "plugin/sign/testdata/db.miek.nl.signed" with serial 1563637748
~~~
Signed-off-by: Miek Gieben <miek@miek.nl>

* Adjust readme and remove timestamps
Signed-off-by: Miek Gieben <miek@miek.nl>

* Comment on the newline
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/sign/README.md
Co-Authored-By: Michael Grosser <development@stp-ip.net>