Document strings.TrimPrefix and TrimSuffix template functions

Closes #148

content/functions/strings.TrimPrefix.md

@@ -0,0 +1,25 @@
+---
+title: strings.TrimPrefix
+description: Returns a given string s without the provided leading prefix string. If s doesn't start with prefix, s is returned unchanged.
+godocref:
+date: 2017-02-01
+publishdate: 2017-02-01
+lastmod: 2017-02-01
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [strings]
+signature: ["strings.TrimPrefix PREFIX STRING"]
+workson: []
+hugoversion:
+relatedfuncs: [strings.TrimSuffix]
+deprecated: false
+aliases: []
+---
+
+Given the string `"aabbaa"`, the specified prefix is only removed if `"aabbaa"` starts with it:
+
+    {{ strings.TrimPrefix "a" "aabbaa" }} → "abbaa"
+    {{ strings.TrimPrefix "aa" "aabbaa" }} → "bbaa"
+    {{ strings.TrimPrefix "aaa" "aabbaa" }} → "aabbaa"

content/functions/strings.TrimSuffix.md

@@ -0,0 +1,25 @@
+---
+title: strings.TrimSuffix
+description: Returns a given string s without the provided trailing suffix string. If s doesn't end with suffix, s is returned unchanged.
+godocref:
+date: 2017-02-01
+publishdate: 2017-02-01
+lastmod: 2017-02-01
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [strings]
+signature: ["strings.TrimSuffix SUFFIX STRING"]
+workson: []
+hugoversion:
+relatedfuncs: [strings.TrimPrefix]
+deprecated: false
+aliases: []
+---
+
+Given the string `"aabbaa"`, the specified suffix is only removed if `"aabbaa"` ends with it:
+
+    {{ strings.TrimSuffix "a" "aabbaa" }} → "aabba"
+    {{ strings.TrimSuffix "aa" "aabbaa" }} → "aabb"
+    {{ strings.TrimSuffix "aaa" "aabbaa" }} → "aabbaa"