add replace function (#306)

Co-authored-by: Felix Maier <fm41@mailbox.org>

README.md

@@ -212,6 +212,9 @@ render("Hello {{ upper(neighbour) }}!", data); // "Hello PETER!"
 render("Hello {{ lower(neighbour) }}!", data); // "Hello peter!"
 render("Hello {{ capitalize(neighbour) }}!", data); // "Hello Peter!"
 
+// Replace characters in a string
+render("{{ replace(neighbour, \"e\", \"3\")}}", data); // "P3t3r"
+
 // Range function, useful for loops
 render("{% for i in range(4) %}{{ loop.index1 }}{% endfor %}", data); // "1234"
 render("{% for i in range(3) %}{{ at(guests, i) }} {% endfor %}", data); // "Jeff Tom Patrick "