Add optional separators

[deeplook]

For long sparklines it would be useful to have separators like : in the output such that e.g. sensor values over a period of several hours can be visually grouped into bins of e.g. four hours. The question might be if these should be described with options or rather with real separators between the values:

$ sparklines -n 2 --grouplen 4 --groupsep ":" 1 2 3 4 5 6 6 5 4 3 2 1
   ▂:▅██▅:▂
▁▄▇█:████:█▇▄▁

$ sparklines -n 2 1 2 3 4 : 5 6 6 5 : 4 3 2 1
   ▂:▅██▅:▂
▁▄▇█:████:█▇▄▁