Added select.cols option to sqliteReadTable (and hence to dbReadTable)

Hi there,

   Just a suggestion but I've found this useful when I need to pull in only the variables I need from a fairly wide database.

   I've checked it with `R CMD check` and some manual testing.  Updated the man page but didn't write any unit tests because, y'know, I'm lazy :-)

Thanks for a great package.

Best,
Ari Friedman