Support for --iseek, --oseek and --count options

[schwesinger]

Transferring only parts of a source image to a specific location in a target device (or destination image) is necessary in use cases where individual partitions are to be copied.
To support this, options for specifying the input file offset, the amount of data to be copied as well as the output file offset are needed.

As bmaptool aims to be a dd replacement these options could be named similar to what dd uses, namely:

• --skip (alias --iseek) for the input file offset
• --seek (alias --oseek) for the output file offset
• --count for the amount

This issue condenses the two related issues intel/bmap-tools#9, intel/bmap-tools#24 from the old repo.

[josch]

That feature would also be a solution to my quest for finding a tool that is like dd but is able to create sparse output files: https://superuser.com/questions/1837708/