Bootstrap: Validate --pre-release argument

Fixes gh-42

Author: jzaefferer

lib/bootstrap.js

@@ -1,5 +1,6 @@
 var fs = require( "fs" ),
 	path = require( "path" ),
+	semver = require( "semver" ),
 	which = require( "which" );
 
 module.exports = function( Release ) {
@@ -41,6 +42,11 @@ Release.define({
 		Release.branch = Release.args.branch || "master";
 		Release.preRelease = Release.args.preRelease || false;
 
+		if ( Release.preRelease && !semver.valid( Release.preRelease) ) {
+			Release.abort( "Invalid --pre-release argument, not valid semver: " +
+				Release.preRelease );
+		}
+
 		console.log();
 		console.log( "\tProject: " + Release.project );
 		console.log( "\tRelease type: " + (Release.preRelease ? "pre-release" : "stable") );