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zx.start() in REPL now preserves the REPL's imports and locals

• Improve support for shell strings inside functions
• Allow accessing attributes on shell strings, like (~"grep ...").splitlines()

Remove the use of := operator for backwards compatibility.

• It no longer waits for the shell command to finish before printing its output. eg. ~'echo hello; sleep 1; echo bye'
• Added REPL support: Running zxpy with no arguments starts up a REPL. More info here.

Simple improvements to the code quality and documentation.