Version 0.1.8

Changes in this version:

• Add 'version' command to get the current fend version
• Support -sin pi and 3 sin pi without parentheses
• Support function inverses: e.g. sin^-1 becomes asin
• Add to sf to convert numbers to a fixed number of significant figures
• Make many calculations involving sin and cos exact
• Add tau (τ), equal to 2π
• Add Yi- and Zi- binary prefixes
• Use decimal approximations instead of fractions by default
• x to exact will now convert x to an exact representation whenever possible, including using multiples of π
• Add cis as a shorthand for cos θ + i * (sin θ)