Don't stop on failure to load a rule, simply skip it

We do, however, stop if no rule was loaded (since there's nothing to do).
Add error when no rules are loading from a file

E.g a file with nothing but "[Rule]" is it
Add options --verbose && --quiet
Some little additions/tweaks to the man page

- mention field variables on trigger
- add example for using shell command line
- mention systemd.journal-fields(7) for a list of special fields
Add support for $'FIELD in trigger
Bump to v0.2.0; Update HISTORY