Add --tags option to pipe and post

The --tags option takes a string with comma-separated values:

    tumblr post --tags "dr who,spoilers" fifteenth-doctor.jpg
    echo http://funny.com | tumblr --tags funny

It is especially useful when posting URLs, audio, image, or
video files; previously, the only way to set tags on these in
a hands-off manner was programmatically or using $EDITOR hacks.