Add support for "Trailing path commands" for Plot2D

[DJDuque]

The general command for a plot is:

\addplot[keys] coordinates {
    % coordinates
} <trailing path commands here>;

How should we include support for this? My first idea is to add Option<TrailingPath> to the Plot2D, where

pub enum TrailingPath {
    Custom(String), // Write your own custom command
    ClosedCycle, // corresponds to "\closedcycle"
    Cycle, // corresponds to "--cycle"
}

But I don't know enough about this.