better file extension support for both server side script and HTML

[imthenachoman]

• Fixed issues with fileExtension (see using anything other than gs, js, or ts for fileExtension results in script file not being pushed #884)

• Added htmlExtension for a custom HTML file extension (see feature request: fileExtension type setting for html #703)

• npm run test succeeds.

• npm run lint succeeds.

• Appropriate changes to README are included in PR.

[google-cla]

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[imthenachoman]

@googlebot I signed it!

[imthenachoman]

@PopGoesTheWza Sorry to bug you but it seems you're the primary for clasp. Is this something y'all have had a chance to look at and will consider incorporating? If not I understand but I was hoping to know either way.

[imthenachoman]

@PopGoesTheWza Anything I can do to help get this merged?

[imthenachoman]

@PopGoesTheWza Just checking on this?