Save frames of the contact sheet

[KaiSchu]

The intention of this pull request is to add a feature to save the frames that are used in the contact sheet in addition.

[coveralls]

Coverage decreased (-0.2%) to 30.625% when pulling 749c68d on KaiSchu:master into 1e43802 on amietn:master.

[amietn]

Thank you for this pull request. Much appreciated.

I will merge but will make a few changes afterwards, namely:

• The -to option shadows the usage of -t and -o together. Will rename it.
• There is no error message if thumbnails cannot be copied, for example if the specified directory does not exist.
• Frames are copied outside of the specified directory if a final slash is omitted.