Added flag to merge videos from a playlist into one continuous file

#### Example Use-Case ####
```
youtube-dl http://thecolbertreport.cc.com/full-episodes/wllm5p/april-30--2014---audra-mcdonald
```
Usually, this produces four videos, one for each segment of the show. However...
```
youtube-dl --concat http://thecolbertreport.cc.com/full-episodes/wllm5p/april-30--2014---audra-mcdonald
```
Will produce one continuous video for the show.