Command-line argument support

Support optional leading + for numbers. #67.
REPL command for deleting symbols from the main context (?del). Closes #70.
Pass command line arguments to Song scripts in an "args" list. Closes #31.
Add number() builtin for converting strings to numbers.

Bug fixes and more consistent equality tests

Permit Boolean equality tests.
Test character equality.
Permit "'" and '"' literals.
Use Song Eq operator for comparing lists, not Swift ==. This means lists containing floats can no longer be compared.

Initial release

v0.1.0

Use shields.io.