Provide helper methods for common options

[tjgq]

The SANE API does not prescribe standard names for most options; however, some of the most common ones can be guessed (see, e.g., how simple-scan [1] does it).

We could provide methods such as SetMode, SetResolution, etc to set these options directly and avoid too much boilerplate.

[1] https://launchpad.net/simple-scan