Allow overriding MIME type, check for stdin input automatically about 0x0 HOT 2 OPEN

I added the --mimetype functionality in #6

Regarding the "auto-detect" whether there is some content in stdin (and not require using - when invoking the script), I am not sure if there is a reliable way to know whether there is or will be some content coming from stdin. Currently if you invoke the script with no arguments it produces the help message, so without that, it would just sit there any wait for input (may be confusing for users). So, I think it is better to explicitly use - to indicate use stdin for the content.

from 0x0.

and i meant, by stdin, that a user doesnt need to specify - on the command line... essentially, the check should be donne here otherwise... what fun is it really :)

from 0x0.