Add support for loading webhook filter JavaScript from external file or URL

Since Lithium Sync Gateway supports externalizing JavaScript function definitions in separate JavaScript files and referencing those files in the Sync Gateway configuration file. An enhancement has already been merged to master which mainly includes sync function, import filter and conflict resolver function definitions. In addition to this we also need to include the JavaScript filter that is being used in Webhook event handlers to examine the contents of the mutated documents and decide which ones to post to the specified URL.