fix: run tasks in correct working directory

[metaist]

Typically, we want to run commands relative to the configuration file. If we ever implement #10, this gets more complicated.

[metaist]

Idea 1: Always change to the directory of the configuration file.

This is highly regular, but it can get weird if you supply a configuration file that's in some totally other project.

[metaist]

Idea 2: Add a flag --cwd PATH that lets you override the current working directory.

Not too bad. Similar to Idea 1, but would let you quickly make the current directory the active one: ds --cwd . create-app