Configurable number of PhantomJS instances from the includer's package.json

After this commit, one who depends on attester in his "scripts" part of
package.json, can additionally put in that file the following:

 "config":{"phantomjsInstances":2}

to specify the number of PhantomJS instances (instead of passing it via
command line), but as the added benefit, the actual user who npm install'ed
his package, could type:

 npm config set somePackage:phantomjsInstances 8

to take advantage of his multi-core machine.

This setting is taken into account only if there was no
--phantomjs-instances passed from the command line when invoking attester.