Make `bst init` take an argument

Background

Currently, bst init will automatically initialize a new project in the current working directory. This is nice and all, but when given an argument, BuildStream will fail.

I think bst init should behave more like git init for familiarity reasons, and use an argument to specify a directory in which to init.

Task description

Make bst init take an argument and initialize a new project in the given directory - behave as currently when no argument is given.

Acceptance Criteria

bst init should accept an argument.

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