Add `all` target and Fisherman installation note.

[ghost]

In the current implementation, running make will run make install by default causing Fisherman to deploy bass to ~/.config/fish/functions when it should not.

Even if Fisherman was able to patch this for the specific case of bass, it seems like a good practice to separate the build and install steps.

tl;dr

• This PR does not break the compatibility with either fundle or the recommended installation method described in the README.
• Add compatibility to Fisherman and add note in README with install instructions using Fisherman.

[edc]

Thanks for contributing this!

[ghost]

@edc Thank you for making this and saving a lot of people so much trouble! :smile: