v0.2.4

• Mount handlers are now called "data sources", a generalization that will make them more pluggable
  and support sources beyond FDWs. See #324 for more documentation and necessary steps to migrate.
• Added sgr singer target, a Singer-compatible target that can read Singer tap output from stdin and build Splitgraph images. It's based on a fork of https://github.com/transferwise/pipelinewise-singer-python with additions that let us produce deltas and ingest them directly as Splitgraph objects.
• Support for dynamically loading plugins without specifying them in .sgconfig, by looking up plugins in a certain directory (see #329)