Add support for more url ARGS:

timepicker: the mode, absolute, relative, since
from: depending on the mode, either time from present, or a date
to: when mode is absolute, the end date/time
style: light/dark

Only added to logstash.json, but if you want, I'll add to logstash.js.
Thinking about how to support saved dashboards, which would make this change unnecessary.