Use absolute address for erase; also support offset

[florisla]

From #73, after much discussion, and with a better title:

• Support absolute address for --erase, to make it consistent with other commands.
• Support --offset as address relative to start of flash.
• If a relative address is used as --address, print an error and exit.
• Support --length also when flashing. Truncate file when smaller; print an error when size is larger than file.
• Support --erase and flash in one command, also with --length.
• For --erase, when --length is given, don't do global erase but only erase the necessary pages.
• Possibly add --global-erase option too.

[kaidegit]

Maybe we can support auto erase the sectors when writing flash.