adding stringToGlyphIndexes()

[axkibe]

Description

adresses #574

How Has This Been Tested?

I tested it with my quite larger app, it works, it's quite a trivial change tough.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
  No I didn't, I tested it with my application.
• My change requires a change to the documentation.
• I have updated the README accordingly.
  charToGlyphIndex() and nameToGlyphIndex() are also not mentenioned in the README, maybe we should add all?
• I have read the CONTRIBUTING document.

[yne]

@axkibe could you add a test case in test/font.js ?

[Connum]

I also reviewed it, other than the missing test as mentioned, it looks good to me!

[axkibe]

Ok I'll add the test tomorrow or the day after.

[axkibe]

Ok, added.