Add Elli support

[oubiwann]

Tasks:

• [x] Copy example from lfex/examples/elli (Github repo)
• [x] Rename handler module to routes
• [x] Convert handle/3 to defroutes macro
• [x] Create include/elli.lfe
• [x] Implement defroutes macro for elli
• [x] Update examples routes to use the new macro
• [x] Ensure example works with manual testing
  • [x] fill in example rebar.config enough to get it running
  • [x] update the README with instructions for running the example in the REPL
• [x] Add unit tests for new functions
• [x] Add integration testing that uses example
  • [x] create a new profile in rebar.config that includes the example source
  • [x] add a new job in CI/CD for running tests against the example
  • [x] ensure xref and dialyzer pass
  • [x] add setup and tear down for starting the example app
  • [x] add integration tests for every route
  • [x] update the README with instructions on running the integration tests against the example (the rebar3 command from CI/CD)

[oubiwann]

Done.