Default path for jupyter-book CLI switched to "./" (rather than required argument)

[mmcky]

Currently the jupyter-book cli requires a PATH to be entered when building books.

Is there a reason we can't set

PATH_SOURCE = "./" (default value)

with an Error if it can't find _config.yml or _toc.yml in the current working directory.

From my perspective it would be nice to be able to issue:

jb build and not jb build ./

as this is similar to a lot of other cli tools such as git that are context aware.

[chrisjsewell]

jupyter-book/jupyter-book#890