Sabard/darwin

[sabard]

No functional changes here, just the following fixes for MacOS:

• base64: use -b 0 (default and equivalent to -w 0)
• csplit: use gcsplit on mac (requires coreutils)
• syntax:
  • [ -v ] -> [ -n ]. MacOS ships with /bin/bash that doesn't support the newer command
  • use cut since substring end lengths don't work in old bash version

Fixes #1

Need to test on Linux before merging.