Sanitize Snap name so it conforms to format requirements

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Make sure that the name given to the Snap conforms to the requirements.

Addresses electron/forge#654

[codecov]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #13   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         227    236    +9     
=====================================
+ Hits          227    236    +9

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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