Add basic string manipulation join and split

Signed-off-by: Knut Ahlers <knut@ahlers.me>

README.md

@@ -47,6 +47,12 @@
   $ echo '{{ hash "sha256" "this is a test" }}' | korvike
   2e99758548972a8e8822ad47fa1017ff72f06f3ff6a016851f45c398732bc50c
   ```
+- `{{ join <string> <delim> }}`  
+  Joins the given slice of strings with given delimiter.
+  ```console
+  $ echo '{{ join (split "a,b,c" ",") "|" }}' | korvike
+  a|b|c
+  ```
 - `{{ markdown <source> }}`  
   Format the source using a markdown parser
   ```console
@@ -59,6 +65,12 @@
   $ echo '{{ now "2006-01-02 15:04:05" }}' | korvike
   2017-04-17 16:27:34
   ```
+- `{{ split <string> <sep> }}`  
+  Splits the given string by given separator.
+  ```console
+  $ echo '{{ range (split "a,b,c" ",") }}{{ . }} {{ end }}' | korvike
+  a b c
+  ```
 - `{{ tplexec (file "my.tpl") }}`  
   Execute the given template with the same function set and variables as the parent template.
   ```console

functions/func_strings.go

@@ -0,0 +1,8 @@
+package functions
+
+import "strings"
+
+func init() {
+	registerFunction("join", strings.Join)
+	registerFunction("split", strings.Split)
+}