Add file extensions and helper functions to meta.js

Added file extensions to the meta modeInfo which can be used to determine which mode should be used.

Added helper functions:
getExtFromMimeType: returns the extension for the given mime type
getMimeTypeFromExt: returns the mime type for the given extension
getFilenameAndExt: returns the base name and extension for the given filename 
detectCodeMirrorMode: returns the codemirror mode name for the given mime type