templates: Add 'import' action (#4321)

Related to (closed) Issue #2094 on template inheritance. This PR adds a new function called "import" which works like "include", except it only takes one argument and passes it to the referenced file to be used as "." in that file.

* Update tplcontext.go

Add {{ render "/path/to/file.ext" $data }} via funcRender

* Update tplcontext.go

* Refactor funcInclude, add funcImport to enable {{block}} and {{template}}

* Fix funcImport return of nil showing up in html

* Update godocs for  and

1 files changed, 17 insertions, 1 deletions

diff --git a/modules/caddyhttp/templates/templates.go b/modules/caddyhttp/templates/templates.go
index 6fed8c8..8f2ea2c 100644
--- a/modules/caddyhttp/templates/templates.go
+++ b/modules/caddyhttp/templates/templates.go
@@ -91,9 +91,25 @@ func init() {
 // {{httpInclude "/foo/bar?q=val"}}
 // ```
 //
+// ##### `import`
+//
+// Imports the contents of another file and adds any template definitions to the template stack. If there are no defitions, the filepath will be the defition name. Any {{ define }} blocks will be accessible by {{ template }} or {{ block }}. Imports must happen before the template or block action is called
+//
+// **filename.html**
+// ```
+// {{ define "main" }}
+// content
+// {{ end }}
+//
+// **index.html**
+// ```
+// {{ import "/path/to/file.html" }}
+// {{ template "main" }}
+// ```
+//
 // ##### `include`
 //
-// Includes the contents of another file. Optionally can pass key-value pairs as arguments to be accessed by the included file.
+// Includes the contents of another file and renders in-place. Optionally can pass key-value pairs as arguments to be accessed by the included file.
 //
 // ```
 // {{include "path/to/file.html"}}  // no arguments