Show only supported data types

Added a factory service which tells whether some given data item can be shown in JBrowse data selector (based on data selector mode and required files). 

- Additional checks for files can be added to _canShowPatterns_ dictionary
- Additional data types for selection modes can be added to _organization_ dictionary

- API:
   __canShow (item, selectionMode)__
   __find(item, propertyPath, regexPattern)__: tests item's property value at given propertyPath against regexPattern and returns the property value if the test was successful. 
_.find(entries, RegExp.test, pattern) || false works beautifully :)