//line and //curve

I've always wondered why WorldEdit didn't have commands like these.

Thanks to TomyLobo for letting me use his interpolation classes from his routes project. Also, his new convex polyhedral regions allowed convenient user input for //curve, and he's given me much advice.

Thanks to wizjany for not pulling this after my first commit, as I'd requested :P

```java
@Command(
    aliases = { "/line" },
    usage = "<block> [thickness]",
    desc = "Draws a line segment between cuboid selection corners",
    help =
        "Draws a line segment between cuboid selection corners.\n" +
        "Can only be used with cuboid selections.\n" +
        "Flags:\n" +
        "  -h generates only a shell\n",
    flags = "h",
    min = 1,
    max = 2
)
```
```java
@Command(
        aliases = { "/curve" },
        usage = "<block> [thickness]",
        desc = "Draws a spline through selected points",
        help =
            "Draws a spline through selected points.\n" +
            "Can only be uesd with convex polyhedral selections.\n" +
            "Flags:\n" +
            "  -h generates only a shell",
        flags = "h",
        min = 1,
        max = 2
    )
```