Feature request: Skip existing files

[NuVanDibe]

Right now, if trying to download multiple videos by ID, and a file exists, it asks File exists. Overwrite? [Y/n]: and if you choose n then it aborts everything. It'd be nice if we had yes once no once yes always no always abort as options, as well as a --skip-existing flag to go along with the --overwrite flag.

[NuVanDibe]

For context, here's the command line I want to get working:

python twitch-dl.2.1.3.pyz download $(python twitch-dl.2.1.3.pyz videos streamername -c | grep -E '[0-9]+[0-9]+[0-9]+[0-9]+[0-9]+[0-9]' | sed 's/ .*//' | tr '\n' ' ' | sed -e 's/\x1b\[[0-9;]*m//g') -q source

[NuVanDibe]

Addressed in PR #138