tpl: Add os.fileExists template function

Fixes #3839

docs/content/functions/fileExists.md

@@ -0,0 +1,29 @@
+---
+title: "fileExists"
+linktitle: "fileExists"
+date: 2017-08-31T22:38:22+02:00
+description: Checks whether a file exists under the given path.
+godocref:
+publishdate: 2017-08-31T22:38:22+02:00
+lastmod: 2017-08-31T22:38:22+02:00
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+signature: ["fileExists PATH"]
+workson: []
+hugoversion:
+relatedfuncs: []
+deprecated: false
+aliases: []
+---
+
+`fileExists` allows you to check if a file exists under a given path, e.g. before inserting code into a template:
+
+```
+{{ if (fileExists "static/img/banner.jpg") -}}
+<img src="{{ "img/banner.jpg" | absURL }}" />
+{{- end }}
+```
+
+In the example above, a banner from the `static` folder should be shown if the given path points to an existing file.