video: support setting libswscale chroma position
m_property: fix non-sense code

Undefined behavior.
command: add pseudo-property that allows you to read global options

The "options" pseudo-property allows reading global like this:

   show_text ${options/name}

Where "name" maps to the option "--name". This allows retrieving option
values that are not properties. Write-access is not possible: this is
reserved for normal properties.

Note: it is possible that we'll change this again, and don't require the "options/" prefix to access options.
command: allow printing time properties with milliseconds

Doing e.g. show_text "${time-pos/full}" will show the time formatted
with a milliseconds part.

This is actually special cased for a few properties which use
CONF_TYPE_TIME, instead of making all properties using CONF_TYPE_TIME
respect this. This is a technical limitation.

I'm not entirely happy with this approach, so I'll leave it
undocumented. It's relatively ok, but he fact that it's special-cased to
some properties is not elegant. So for now, this is just a hack to make
ChrisK2 happy (hi there).