Add support to `Install-ChocolateyFileAssociation` for multiple file extensions

[philoushka]

Moving discussion from the old chocolatey issue 683 for function Install-ChocolateyFileAssociation

Summary

• allow callers to send multiple file extensions instead of calling this method n times for n file extension associations.
• change param string $extension to string[] $extensions
• existing code will out in the wild will work because of parameter name disambiguation. extension is sufficient enough to be disambiguated to extensions, just as ext would too.
• change function to iterate through $extensions one by one and use existing code with no changes

[jberezanski]

A minor nitpick: PowerShell convention for parameter naming is to always use singular nouns, even in case of arrays. Therefore, I'd recommend against the name change.

[pauby]

Extension should be able to take a string or an array of strings. I agree with @jberezanski, PowerShell uses singular nouns and the parameter shouldn't be renamed.

Any change made to the PowerShell helper will also need to be made in Chocolatey Licensed.