CallRecording module cant handle dialplan vars

the CallRecording module allows the ability to call variables such as MIXMON_POST, MONITOR_REC_OPTION, which are stored in `/etc/asterisk/globals_custom.conf`

However, at present it does not properly evaluate them.

Specifically, a basic script of

echo "Hello!" >> /tmp/postrecording.txt

and setting globals_custom.conf > Mixmon_post=/path/to/script.sh WILL successfully execute the command when a recording has completed, but if you want something more complex (i.e. `

MIXMON_POST = /usr/bin/mix-stereo.sh ${SS}{MIXMON_DIR}${SS}{YEAR}/${SS}{MONTH}/${SS}{DAY}/ ${SS}{CALLFILENAME}

 it does not evaluate the contents of items such as {YEAR} from the dialplan.

at present, the usage of MIXMON_POST is restricted and unnecessarily restricted, and this issue / pull request serve to greatly expand the usability of the tool.