Allow VRT ids to contain numbers (#17)

The current validation fails on validating VRT IDs that contain numbers
which breaks on the current version of VRT. This commit loosens the
validation on IDs to allow digits.

Author: Devin Riley

lib/vrt/map.rb

@@ -42,7 +42,7 @@ def categories
 
     def valid_identifier?(vrt_id)
       # At least one string of lowercase or _, plus up to 2 more with stops
-      @_valid_identifiers[vrt_id] ||= vrt_id =~ /other|\A[a-z_]+(\.[a-z_]+){0,2}\z/
+      @_valid_identifiers[vrt_id] ||= vrt_id =~ /other|\A[a-z_\d]+(\.[a-z_\d]+){0,2}\z/
     end
 
     def construct_lineage(string, max_depth)

spec/vrt/map_spec.rb

@@ -121,5 +121,10 @@
       let(:string) { 'server_security_misconfiguration.meep_meep_meep' }
       it { is_expected.to be_falsey }
     end
+
+    context 'when contains numbers' do
+      let(:string) { 'sensitive_data_exposure.token_leakage_via_referer.untrusted_3rd_party' }
+      it { is_expected.to be_truthy }
+    end
   end
 end