Add bookmarking commands

Note: Releases no longer include binaries. AFAIK they're note being used so this shouldn't be an inconvenience. Feel free to submit an issue if you do need prebuilt binaries.

This release adds bookmark commands:

'cvcap add-bookmark ' adds a bookmark (a checkvist list or task link)

'cvcap add -b ' then adds task to the list or as a child
of the task, rather than to the default list
#27

See help and/or README for details