option to use usermedia constraints

[bastecklein]

This change allows the user to specify a constraints object when calling open() on a UserMedia object, as requested in the following feature request:

#816

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.96%. Comparing base (3f22ce5) to head (6cc3166).

Files with missing lines	Patch %	Lines
Tone/source/UserMedia.ts	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1330      +/-   ##
==========================================
+ Coverage   98.95%   98.96%   +0.01%     
==========================================
  Files         203      203              
  Lines       22690    22697       +7     
  Branches      997      997              
==========================================
+ Hits        22452    22461       +9     
+ Misses        238      236       -2     

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