Exclude read-only filesystems from check-disk-usage.rb

[DrMurx]

check-disk-usage.rb usually checks all mounted filesystems unless specific ignore/include options are given. However, it doesn't make a lot of sense to check read-only filesystems. My PR #112 addresses this with a new option --ignore-readonly.

However, I'd strongly vote for a breaking change and ignore read-only filesystems by default unless a new parameter --include-readonly is given or such a filesystem's mountpoint is explicitly specified in the -I (:includemnt) option.