Add an option to set the paragraph line-height by px

[ftessier]

When text blocks contain text of various sizes, the line heights becomes non-uniform as a result. This could be solved by adding an option to specify an exact and fixed line-height by px instead of by "lines" (a factor of the font size on every line). By way of example, when using inline equations, I must set the equation span text size to 120% to match the surrounding text x-height. It would be very useful in this kind of situation to specify a fixed px line-height.