Add browerslist config

This specifies the build targets for Babel's "env" preset, being used to
decide which compatibility transforms will be used in the release file.

This change removes the need for specific Babel plugins, as the build
will analyze the code and automatically add the necessary polyfills and
transforms.

Author: Sebastian Kippe

.browserslistrc

@@ -0,0 +1,6 @@
+# Browsers that we support
+
+cover 99.5%
+ie >= 11
+maintained node versions
+not dead