add knowledge manipulation commands

[ivg]

We need commands to store the knowledge base (in the user-specified location) and then to load it, as well as an injection point for registering knowledge base analyses. The idea is that the binary could be analyzed, e.g., using the disassemble command,

bap disassemble /bin/ls --knowledge-base=ls.kb

and later the stored KB could be loaded easily and immediately analyzed, e.g.,

bap analyze --knowledge-base=ls.kb

One of the UX questions is how to deal with the caching. Right now we store and load the knowledge bases in the cache and we probably don't want to store/load it twice. And what is the best UX for specifying KB locations and whether they should be loaded, stored, or both.