Allow exporting the whole page
Let error() exit and simplify program flow
Add custom "boolstr" parser option type
Simplfiy density parsing
Add export() method for all densities
Write into devnull instead of capturing output
Rename attribute name to "ids"
Let --source value be validated by optparse
Separate ImageMagick and OptiPNG usage switches
Merge pull request #1 from bfd/master

Many optimizations by bfd