Allow preferences to be overridden by a configuration file.

src/extensions/default/JavaScriptCodeHints/Preferences.js
    New class to parse JSON preferences object and store preferences.
src/extensions/default/JavaScriptCodeHints/ScopeManager.js
    Look for preference file, .jscodehints, in the project root directory.
    Remove obsolete "shouldPrimePump" parameter.
src/extensions/default/JavaScriptCodeHints/main.js
    Remove obsolete "shouldPrimePump" parameter.
src/extensions/default/JavaScriptCodeHints/unittests.js
    Add new preference tests.

Add new preference tests under unittest-files/preference-test-files
preferences for 'excluded-directories' and 'excluded-files' always append to the builtin defaults
Merge pull request #4147 from dloverin/pref-file

Allow preferences to be overridden by a configuration file.