add timezones support

[crispin-kirchner]

• add ability to pass TZIDs to start, end and exclusionDates
• add ability to include time zone definitions in the header. I assume that these are downloaded from elsewhere, therefore they are considered well-formed and are put as-is into the output
• the responsibility to provide time zone information for each time zone used in events is with the user, there is no validation which checks whether there is a VTIMEZONE for each TZID

fixes #198

[terreng]

I found one small issue with this PR: TZID shouldn't be added with VALUE=DATE, per the spec

The "TZID" property parameter MUST NOT be applied to DATE properties - Source

Here's how I corrected it: terreng/ics@build-ae3533a...build-3d177a5

Also, while this PR helpfully updates the README to show how to get VTIMEZONE blocks using tzurl.org, for anyone looking to do it locally & synchronously, here's how you can use timezones-ical-library:

import { tzlib_get_ical_block } from 'timezones-ical-library';

...

ics.createEvents(events, {
  timezones: timezones.map(timezone => tzlib_get_ical_block(timezone)[0]).join('\n')
})