tpl: Add float template function

Add a template function that allows conversion to float. This is
useful, for example, when passing aspect ratios into templates,
which tend to not be integers.

Fixes #3307

docs/content/functions/float.md

@@ -0,0 +1,26 @@
+---
+title: float
+linktitle: float
+description: Creates a `float` from the argument passed into the function.
+godocref:
+date: 2017-09-28
+publishdate: 2017-09-28
+lastmod: 2017-09-28
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [strings,floats]
+signature: ["float INPUT"]
+workson: []
+hugoversion:
+relatedfuncs: []
+deprecated: false
+aliases: []
+---
+
+Useful for turning strings into floating point numbers.
+
+```
+{{ float "1.23" }} → 1.23
+```