support cli argument to change the bookmark collection

[sassman]

As of now curlz reads + writes a bookmark collection to $CWD/.curlz.
A new cli argument should allow changing the folder that is used to look up the .curlz folder.

The default value for this argument should be CWD to allow for a non-breaking change.
This argument needs to be supported for the sub-command request and bookmark, hence it might be a good idea to make it a global, sub-command independent argument.