some arguments passed to convert need to precede the source filename

[jsierles]

Some arguments, such as 'density' for specifying a source PDF file's resolution, must come before the source filename, otherwise they are ignored. This is important for controlling the target resolution of a vector image converted to PNG.

BAD % convert test.pdf -density 150 test.png

GOOD % convert -density test.pdf test.png

Should a PR for this should include an extra option for these arguments?