DuckPAN: change --no_check to --check.

Also, make is negativeable so that --no-check works.  This is a more
standard way of doing these kind of options.
DuckPAN: add option to turn on/off colored output

Defaults to colors on, can be switched off with --no-colors.

This doesn't work so well with the Web portions, which don't have access
to duckpan switches.

Fixes #29.
Simplify cache handling.

- Change 'empty' command to '--empty' switch.
- Remove '--force' from Server.
- Remove '--cachesecs' from Server.

This allows other commands (say, `query`) to benefit from the same
cache-handling as the `server` command.

If we just empty the cache up-front then we don't have to keep checking
to see if there was a force.  We can just see if they exist and are new
enough.
DuckPAN: make --no-check turn off all checking.

If this is not what is intended, we should probably rename the switch.

Also, do an empty_cache in check to more generally require a
full re-check.
Add doc strings for existing commands.

TshHey can be seen with --help.. which feels quite conflicting with the
'help' command.  Not sure what to think about that.
Merge pull request #174 from duckduckgo/mwm/options

Options updates.