Add support for "economy" decomposition.

[ignirtoq]

For m-by-n matrices with n < m, the "economy" decomposition computes only the first n columns of Q and n rows of R. When n >= m, the economy decomposition is equivalent to the usual decomposition.

This is the functionality provided by passing in the string 'econ' or the number 0 to the MATLAB qr function described here: https://www.mathworks.com/help/symbolic/qr.html

[yig]

This is good. Can you please update the documentation (docstring and example in the readme)? Also, I left a comment about using None as the default value of economy.

[ignirtoq]

I have modified the default value and updated the documentation.

[yig]

Great! I merged this change.