Allow customization of output filenames

[Breakthrough]

Currently, output filenames follow a fixed format: {VIDEO_NAME}.DSME_{EVENT_NUMBER}.avi

As brought up in #88, it would be useful to allow users to specify a custom template. Some of the template variables that might need to be supported are:

• video name
• event number
• event start time / frame number
• event end time / frame number

TODO:

• [ ] Figure out a better syntax for specifying output templates - are braces good enough, e.g. {EVENT_NUMBER}?
  • PySceneDetect used $ which was a mistake since it's used for bash variables, braces seem well supported enough
• [ ] How to deal with precision (both leading and trailing zeros)? Is a fixed number good enough for now?
  • Examples: For the event number, if the template is {EVENT_NUMBER}, allow an integral suffix, e.g. {EVENT_NUMBER_3}
  • For timecodes, e.g. {EVENT_START_TIME}, allow a suffix with the precision of seconds, e.g. {EVENT_START_TIME_3}