Integrate & adapt selectFiles function

[runegan]

Original report by Zack Lovatt (Bitbucket: zlovatt, GitHub: zlovatt).

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Based on this by @rafikhan, adapted from something by @rendertom

Needs to be aequery-ified, and implemented as aeq.file.selectFiles();

var myFiles = selectFiles(["jsx", "theme", "someOtherExtension"], true);

function selectFiles(extensionList, multiSelect)
{
    var message = multiSelect ? "Please select multiple files" : "Please select file";

    var getFilterForFiles = function ()
    {
        var isWindows = ($.os.indexOf("Windows") != -1);

        if (isWindows) {
            return '*.' + extensionList.join(';*.');
        } else {
            var extensionListRe = '\.(' + extensionList.join('|') + ')$';
            var re = new RegExp(extensionListRe, "i");

            return function (file) {
                return file.name.match(re) || file.constructor.name === "Folder";
            };
        }
    }

    return File.openDialog(message, getFilterForFiles(), multiSelect);
}

[runegan]

Original comment by renderTom - (Bitbucket: [Tomas Sinkunas](https://bitbucket.org/Tomas Sinkunas), ).

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Add third optional 'message' parameter to the function, so you can prompt for custom message when dialog appears.

[runegan]

Original comment by Zack Lovatt (Bitbucket: zlovatt, GitHub: zlovatt).

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Adds selectFiles to aeq.file (resolve #31)