Filter an icalendar file based on class and/or category
ical2html [\,options\/] \,input output\/
-p, --class=\,CLASS\/
only (PUBLIC, CONFIDENTIAL, PRIVATE, NONE)
-P, --not-class=\,CLASS\/
exclude (PUBLIC, CONFIDENTIAL, PRIVATE, NONE)
-c, --category=\,CATEGORY\/
only events of this category
-C, --not-category=\,CATEGORY\/
exclude events of this category
input and output are iCalendar files