Allow creation of arbitrary labels (address, basic_block, etc)

[leishen]

I'd like to request labels for basic blocks. If I have a jump to an address, I'd like to be able to label the address and have that label displayed in the beginning of the block. Similarly, I'd like to be able to add labels for any block whether it has a reference or not. I don't think labels are necessary at the sub-block level, but I do not have a strong opinion on that one way or another.

[crowell]

dup of #75 ?

[psifertex]

I think it's slightly different -- this one is probably best categorized as "allow creation of labels anywhere" which would be a distinct feature from #75 which is probably "add default label to each basic block with address".