Make 'scope' an optional option

[Smona]

First off, I'd like to thank the maintainers for the lovely little hotkey library! I ran into some difficulty when writing a Typescript wrapper for hotkeys due to the fact the scope option is required.

The documentation doesn't mention this as a required option, and in use the library works fine passing an options object without scope, so it should probably be marked as optional.

[codecov-io]

Codecov Report

Merging #95 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #95   +/-   ##
======================================
  Coverage    83.9%   83.9%           
======================================
  Files           1       1           
  Lines         174     174           
======================================
  Hits          146     146           
  Misses         28      28

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[coveralls]

Pull Request Test Coverage Report for Build 156

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 70.942%

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Totals
Change from base Build 154:	0.0%
Covered Lines:	146
Relevant Lines:	174

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💛 - Coveralls