Support semver checks

test/unit/interface.js

@@ -2,6 +2,7 @@ const fs = require('fs')
 const path = require('path')
 const yaml = require('js-yaml')
 const ergogen = require('../../src/ergogen')
+const version = require('../../package.json').version
 
 // fixtures
 const load = name => yaml.safeLoad(fs.readFileSync(
@@ -56,4 +57,13 @@ describe('Interface', function() {
         ])
     })
     
+    it('engine', async function() {
+        return Promise.all([
+            ergogen.process({'meta.engine': 'invalid'}).should.be.rejectedWith('Invalid'),
+            ergogen.process({'meta.engine': '^0.1.2'}).should.be.rejectedWith('satisfy'),
+            // no "points clause" means we're over the engine check, so it "succeeded"
+            ergogen.process({'meta.engine': `^${version}`}).should.be.rejectedWith('points clause')
+        ])
+    })
+    
 })