fix: math filters now return number, resolves #110

* fix linting
* numbers are no longer fixed:
    i.e. {{0.3 | minus 0.1}} now renders as 0.19999
    Stick to JavaScript number behavior, see: 8.0.0

test/integration/liquid/liquid.ts

@@ -1,8 +1,10 @@
 import Liquid from '../../../src/liquid'
 import * as chai from 'chai'
 import { mock, restore } from '../../stub/mockfs'
+import * as chaiAsPromised from 'chai-as-promised'
 
 const expect = chai.expect
+chai.use(chaiAsPromised)
 
 describe('Liquid', function () {
   describe('#plugin()', function () {
@@ -34,6 +36,12 @@ describe('Liquid', function () {
       const html = await engine.parseAndRender(tpl)
       expect(html).to.equal('Welcome(to]Liquid')
     })
+    it('should support for-in with variable', async function () {
+      const src = '{% assign total = 3 | minus: 1 %}' +
+        '{% for i in (1..total) %}{{ i }}{% endfor %}'
+      const html = await engine.parseAndRender(src, {})
+      return expect(html).to.equal('12')
+    })
   })
   describe('#express()', function () {
     const liquid = new Liquid({ root: '/root' })