Add support for parsing an rrule string without frequency

[martinjlowm]

Prior to this PR, rrulestr fails to parse a simple RRule of "DTSTART:X".

• As an RRule, rrulevals becomes empty and groomRruleOptions spreads an undefined value.
• As an RRuleSet, dtstart is parsed and only ever used if the parsed RRule string contains a frequency component ("RRULE:Y").

This PR stores the parsed DTSTART as a field on an RRuleSet which may be changed using the new dtstart-setter. The DTSTART component is only printed if RRuleSet._dtstart is set and RRuleSet._rrule is an empty array to avoid conflicting outputs.

• Merged in or rebased on the latest master commit
• Linked to an existing bug or issue describing the bug or feature you're
  addressing (- I hope the description here is enough)
• Written one or more tests showing that your change works as advertised
• Run yarn build to rebuild the dist/ files

edit:

FYI, with the changes in e1146a3, RRuleSet.tzid() and RRuleSet.dtstart() lose their type signatures. These may be recovered by upgrading TypeScript to 3.2+ with the strictBindCallApply-option.

[codecov-io]

Codecov Report

Merging #339 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
+ Coverage   89.92%   90.01%   +0.09%     
==========================================
  Files          28       28              
  Lines        1935     1953      +18     
  Branches      581      583       +2     
==========================================
+ Hits         1740     1758      +18     
  Misses        195      195

Impacted Files	Coverage Δ
src/rruleset.ts	100% <100%> (ø)	⬆️
src/rrulestr.ts	92.56% <100%> (+0.06%)	⬆️
src/iter/poslist.ts	100% <0%> (ø)	⬆️
src/masks.ts	100% <0%> (ø)	⬆️

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[davidgoli]

Hi! What's the use case for this that isn't supported by RDATE?

[martinjlowm]

Hi :)

To support something as simple as rrulestr('DTSTART:19970902T090000Z') and to recover the original string as follows

rrulestr('DTSTART:19970902T090000Z', { forceset: true }).toString() === 'DTSTART:19970902T090000Z'

[davidgoli]

This seems to work OK without altering existing behavior. Thanks for the submission!