Add exec (#10)

* Add exec

* Fix linux tests

* unnecessary dependency

* Dont prefix ExecOptions with I

* Consistency nits

* Respond to feedback

* Add toolrunner explanatory quote

* Format

README.md

@@ -0,0 +1,7 @@
+# `@actions/exec`
+
+> Functions necessary for running tools on the command line
+
+## Usage
+
+See [src/exec.ts](src/exec.ts).