prefix() proposal. Closes broadinstitute/cromwell#375

SPEC.md

@@ -95,6 +95,7 @@
   * [Pair(X,Y) zip(X,Y)](#pairxy-zipxy)
   * [Pair(X,Y) cross(X,Y)](#pairxy-crossxy)
   * [Integer length(Array\[X\])](#integer-lengtharrayx)
+  * [Array\[String\] prefix(String, Array\[String\])](#arraystring-prefixstring-arraystring)
 * [Data Types & Serialization](#data-types--serialization)
   * [Serialization of Task Inputs](#serialization-of-task-inputs)
     * [Primitive Types](#primitive-types)
@@ -2548,6 +2549,17 @@ Array[String] zs = [ ]
 Integer xlen = length(xs) # 3
 Integer ylen = length(ys) # 3
 Integer zlen = length(zs) # 0
+
+## Array[String] prefix(String, Array[String])
+
+:pig2: [Cromwell supported](https://github.com/broadinstitute/cromwell#wdl-support) :white_check_mark:
+
+Given a String and an Array[String], the `prefix` function returns an array of strings comprised of each element of the 
+input array prefixed by the specified prefix string.  For example:
+
+```
+Array[String] env = ["key1=value1", "key2=value2", "key3=value3"]
+Array[String] env_param = prefix("-e ", env) # ["-e key1=value1", "-e key2=value2", "-e key3=value3"]
 ```