Usage:
Use "/sign" to toggle the editing mode.
If you hit a sign with a sign in your main hand, its content will be copied and you can place a new sign with the copied content. If you switch the mode off again you can place the sign again without the copied content.
If you want to change a line of the copied sign, you can use "/sign <line> <content>". The line numbers are 1-based, i.e. with "/sign <1> <Content>" the top line of the sign is edited.

Permissions:
signedit.sign.command
signedit.sign.color