cargo fuzz add should imply cargo fuzz init

[jyn514]

Here's what happens today if you run cargo fuzz add my-fuzzer in a new project:

; cargo fuzz add decompressor
Error: could not read the manifest file: /home/jnelson/work/redacted/fuzz/Cargo.toml

Caused by:
    No such file or directory (os error 2)

The workaround is to run cargo fuzz init first, but it would be nice if cargo fuzz add did that automatically.