Minor improvement to the "memory find" command

Remove the --do-read option, and always provide a small dump of memory at each match spot
Add a --dump-offset (-o) option, to specify a byte offset from which to start dumping relative to the matching address

The real solution is to actually provide the format options found on "memory read" and use those as the key to actually printing memory upon each find
That, however, requires a little refactoring work, so put this in for now until I get a chance to do the required shuffling around of moving parts

llvm-svn: 194600