`as.data.frame.maze()` to help plot with {ggplot2}

[trevorld]

• Adds a as.data.frame.maze() method to help plot with {ggplot2} #1
• Updates {roxygen2} settings to allow markdown e.g. can now use [name_of_function()] to create a cross-reference to name_of_function and `code` to generate \code{code} etc.
• Code example:

x <- maze(12L, 12L)
dfw <- as.data.frame(x, walls = TRUE)
dfs <- as.data.frame(x, start = "top", end = "bottom")
if (require("ggplot2", quietly = TRUE)) {
   ggplot(dfw, aes(x, y)) + geom_tile() + coord_equal() + theme_void() + 
   geom_path(aes(x=x, y=y), colour = "red", linewidth = 2, data = dfs)
}

[trevorld]

Not finished yet since I still plan to add support for method = "geom_segment" but currently provides a working proof of concept.

[trevorld]

• @kstreet13 , this PR for #1 should be ready for review
• In tests/testthat/test-ggplot2.R I added five {ggplot2} snapshot tests plus I added one example in the docs for as.data.frame.maze()
• I also re-organized the existing plotting and printing tests as "snapshot" tests
• I also found it necessary to add a new by argument to solve_maze()
• It would be cool to also add support for geom_polygon() but i think that is beyond the scope of this PR