Add command line option for write and read block size

[EmbeddedMan]

Some chipKIT boards, especially those that are being programmed over an RF link, benefit from smaller packet sizes. Since forcing all boards to use a smaller packet size not only slows them all down, but is also incompatible with some versions of the bootloader (that require 256 byte blocks - see issue #36 ), a better option would be to add a command line parameter (or a pair of them - one for write and one for verify) that allows the user to choose their packet sizes at run time. The boards that are using RF links can then easily change their programming recipe to include the smaller packet size parameter.