Add a tool shim that disables `strip`

[woodruffw]

Lots of build systems invoke strip to remove debug and other sections from the compiled product. Sometimes we want to keep those around even if the build system tries its hardest to remove them, so we should allow the user to inject a fake strip shim into their PATH to disable stripping.

Longer term: we should make Strip into a full fledged blight.Tool and provide a model for it. Then, we can provide a NopStrip or similar action that causes blight to skip any actual strip invocations. The latter will require #32.