Support subfont --formats foo,bar and fix weirdness with --formats consuming further non-option arguments

[papandreou]

I was kinda surprised that subfont --format woff,woff2 didn't work. Turns out yargs doesn't support that even though the option is declared as array: yargs/yargs#846

Also, the array type is weird, so I found that subfont --formats woff2 path/to/page.html fails because yargs tries to consume path/to/page.html as another format 🤪

This PR fixes both issues by switching to string and coercing the result into an array.

[coveralls]

Pull Request Test Coverage Report for Build 557

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 90.212%

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Totals
Change from base Build 550:	0.02%
Covered Lines:	1148
Relevant Lines:	1222

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💛 - Coveralls

[papandreou]

I'll just merge this and get it released.

[papandreou]

Released in 5.2.1.