altTimeFormat doesn't allow an empty value

[sporifolous]

I think it would be advantageous to allow an empty string value for the 'altTimeFormat' option. This would allow putting just the date in the alt field and just the time in the regular field. Currently, if an empty string is passed for 'altTimeFormat', the time is still placed in the altField.

An easy fix: an else if on line 893:

else if (this._defaults.altTimeFormat !== ""){
    altFormattedDateTime += this.formattedTime + altTimeSuffix;
}

I hope this can be helpful.

[ScottDennison]

Note that the same problem applies to altSeparator and altTimeSuffix

[trentrichardson]

I updated the dev branch with a potential fix