(Github Image Viewer) regex pattern improvements + directory detection

• with regexp: add .bmp format (e.g. https://github.com/quentinmrzt/Darlizot/tree/master/sprite), escape filetype dots and ignore the case (e.g. https://github.com/PeterLawrence/WinTimeStampUtility)

--- _imageRegex: /(.jpe?g|.png|.gif|.ico|.tiff?)$/,
+++ _imageRegex: /(\.jpe?g|\.png|\.gif|\.bmp|\.ico|\.tiff?)$/i,

• with folders: check icon sibling for '.octicon-file-text' / !'.octicon-file-directory' class (e.g. https://github.com/github/octicons)

if (target.classList && target.classList.contains("js-directory-link")
+++     && target.parentElement.parentElement.parentElement.firstElementChild.firstElementChild.classList.contains("octicon-file-text")
        && GithubImageViewer._imageRegex.test(target.href)) {