Add incremental build support

[aengelberg]

(Fixes #38)

lein-monolith can now store "fingerprints" for each project, which make a best-effort attempt to cheaply capture when a project changes between builds. They are keyed under user-defined "markers" (e.g. build, test) to distinguish the types of incremental builds we want to keep track of.

• lein monolith each :refresh <marker> ... to perform an incremental build.
• lein monolith changed, lein monolith mark-fresh, or lein monolith clear-fingerprints to inspect and manually manipulate the underlying fingerprints.
• each no longer fails on an empty project selection, to more safely support incremental builds.

[aengelberg]

Ready for another :eyes: @greglook

[aengelberg]

My plan was to cut a snapshot version first so I can sanity-check my assumption that this feature fits well into our CI pipeline before officially releasing it.