Improved defined_integral to accept any single variable function.
Initial lexer implementation.
Added unit tests for lexer and fixed a bug.
Implemented the parser, with basic unit tests.
Also changed the lexer to return an input terminator token at the end.
Updated README.md.
Added tests bad expressions.
Implemented function semantics with unit tests.
Removed support for parsing as integers (they are simply parsed as floats now).
Adjusted parser test cases to test only the parser itself.
Added support for dynamic variable name for the function expression.
Updated README and argparse text.
The main program now supports functions other than the power function.