Add "examples" flag to optionally build examples and tutorials

[fjvallarino]

Add examples flag to optionally build examples and tutorials. By default the flag is False, so examples + tutorials are not built. This is in contrast to the current behavior, where examples and tutorials are always built. In order to build the examples, users need to call:

stack build --flag monomer:examples

Also add a small dev-test-app executable. Used to simplify development/testing of the library itself, since it allows to launch it with ghcid and have changes to Monomer reloaded automatically. Ideally, a temporary solution.