Ask Before Creating an elm-package.json file

[maxsnew]

The current behavior is bad because it doesn't tell the user it's going to create a file and the user may have input the command in error.

The interaction should be

1. Inform the user they don't have an elm-package.json and they need one to use elm-make
2. Ask if they want to make one
3. If yes, either interactively build one or tell them you put a default one in
4. Exit the program

the user can then fill it in and then install things the second time.

[mgold]

+1, it felt very weird to say no to an install but have something created anyway.

[dynajoe]

+1 I'm tired of mistakenly creating elm-stuff and elm-package.json

[evancz]

Tracked in #98