Add Include Captures With String.split as an elixir til

elixir/include-captures-with-string-split.md

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+# Include Captures With String.split
+
+The
+[`String.split/3`](http://elixir-lang.org/docs/stable/elixir/String.html#split/3)
+function comes with two options: `trim` and `parts`. However, when using it
+with a regex pattern, you gain access to a couple extra options, including
+`include_captures`. This is because when used with a regex pattern,
+`String.split` just invokes `Regex.split` which comes with extra options
+like `include_captures`.
+
+Here is `String.split` in action by itself and with the supported `trim`
+option.
+
+```elixir
+> String.split("23d", ~r/\d+/)
+["", "d"]
+> String.split("23d", ~r/\d+/, trim: true)
+["d"]
+```
+
+Adding in the `include_captures` option, we get a resulting list that
+includes the value captured by the splitting regex.
+
+```
+> String.split("23d", ~r/\d+/, trim: true, include_captures: true)
+["23", "d"]
+```
+
+This isn't a documented feature, so it is only supported as long as
+`Regex.split` supports it and as long as `String.split` continues to
+delegate to `Regex.split`.
+
+h/t Chris Erin