Create a command to add an entry to an info fetcher

Right now we have a pluggable info fetcher system whereby fetcher by default can load an overloaded drush aliases file or with the help of fetcher services site load the data from a webservice.

What we need is a command that can interactively build up and write the drush alias file. Where the command will ask specific questions that correspond to settings for the drush alias file or take a list of parameters.

I think that means a new interface for InfoFetcherWriteable or something and then a separate command. The tricky bit will be figuring out what data we need to request and how it might vary depending on the plugins in use (and do we let you select a plugin or have some default).