Add ability to pass arbitrary -o options to sshfs call

[yorinasub17]

This adds a generic option to pass arbitrary -o options to the sshfs call, which allows more flexibility beyond just passing in the idmap flag. E.g., I am using the following to allow bind mounting from local to remote docker host:

sshocker \
  --sshfs-option allow_other \
  --sshfs-option 'idmap=file' \
  --sshfs-option 'uidfile=/home/remoteuser/.sshocker/uidmap' \
  --sshfs-option 'gidfile=/home/remoteuser/.sshocker/gidmap' \
  # ... other args omitted for brevity ...

Related: #31

[AkihiroSuda]

Thanks