Add the ability to build a specific target, instead of everything at once

[jyn514]

Right now, Ekam's only rule is "build everything you know how to build" (where "how to build" is determined by .ekam-rule files). This is great for building binaries, but makes feedback slow for tests, especially given #60. It would be nice to tell Ekam to only build what's necessary for a single test.

This also helps when build tasks are more expensive and take longer to fail than normal, e.g. when building with LTO. If Ekam allowed building only that binary, it would be much faster than also building every other binary with LTO (especially since LTO does most of the work at the link stage, so there's lot of repeated work).

[kentonv]

This is, of course, only possible if Ekam has knowledge of the dependency tree from a previous build. Otherwise Ekam's only way of discovering dependencies is to "explore" and build everything.