Add support for "Trailing path commands" for Plot2D

The general command for a plot is:
```
\addplot[keys] coordinates {
% coordinates
} ;
```
How should we include support for this? My first idea is to add `Option` 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.