Add stitches utilities

[pvalentim]

Includes often used stitches utilities:

• m and p short hand properties
• size shorthand for width and height #15
• Fluid typography using fluidFontSize
• Fluid padding using f prefix on all padding properties.
• appearance for cross browser reset of form control styles
• userSelect for cross browser user selection styles

[pvalentim]

I'm going to merge this since there were no comments