get-bc doesn't parse arguments in the same way extract-bc does

[ianamason]

I added a comment to the effect in the code. I think at some stage
we should build three executables, and when we do that get-bc
can use go's own command line parsing package.

[ianamason]

so now we can use go's "flag" package to do this better. on my TODO list.

[ianamason]

still doesn't mimic extract-bc exactly, but does the best that the flag package allows.