Add a Makefile

[hawkw]

We are quite capable of building everything with cargo, but it might be nice to have an optional Makefile to automate complex stuff or tasks that require multiple cargo commands.

Some things I'd like to see in the Makefile:

• cargo test && cargo test --features unstable for testing with and without nightly-only features
• cargo rustc --features clippy -- -Z no-trans -Z extra-plugins=clippy to lint (not build) the project with clippy and with the compiler's lints
  • clippy would have to be added as an optional dependency for this to work
  • this could be a make lint task that also lints with cargo fmt -- --write-mode=diff if rustfmt is installed
• a make task for doing the coverage task from the Travis config?

Unless we decide to split the rope and editor core repos (#59), the Makefile would need to handle cd-ing into the an-rope directory to run commands for the Rope crate.

[twisted-pear]

i can do it but i'm gonna wait until we've decided on #59, splitting out the rope would make this easier

[hawkw]

@twisted-pear yeah, that seems reasonable!