Add better colors to hexagon and pentagon

[MatthewPeterKelly]

Update the gallery to include at least one image for each class of fractal that the library supports. Also add a few minor readme fixes.

[MatthewPeterKelly]

"Serpinsky" fractal, but generalized and rendered as a hexagon with some nice colors.

{
  "Serpinsky": {
    "fit_image": {
      "resolution": [1000, 1000],
      "padding_scale": 1.05
    },
    "sample_count": 20000000,
    "rng_seed": 12345,
    "subpixel_antialiasing": 2,
    "background_color_rgb": [14, 14, 14],
    "vertex_colors_rgb": [
      [39, 112, 155],
      [132, 92, 162],
      [223, 73, 119],
      [237, 81, 40],
      [228, 191, 48],
      [84, 190, 58]
    ]
  }
}

[MatthewPeterKelly]

Here is an example from the Julia set fractal:

{
  "Julia": {
    "image_specification": {
      "resolution": [1600, 900],
      "center": [0, 0],
      "width": 3.8
    },
    "constant_term": [-0.4, 0.6],
    "convergence_params": {
      "escape_radius_squared": 4.0,
      "max_iter_count": 24000,
      "refinement_count": 5
    },
    "color_map": {
      "keyframes": [
        { "query": 0.0, "rgb_raw": [20, 20, 20] },
        { "query": 0.75, "rgb_raw": [24, 24, 24] },
        { "query": 0.92, "rgb_raw": [39, 112, 155] },
        { "query": 0.93, "rgb_raw": [132, 92, 162] },
        { "query": 0.95, "rgb_raw": [223, 73, 119] },
        { "query": 0.975, "rgb_raw": [237, 81, 40] },
        { "query": 0.99, "rgb_raw": [228, 191, 48] },
        { "query": 1.0, "rgb_raw": [84, 190, 58] }
      ],
      "lookup_table_count": 2048,
      "background_color_rgb": [14, 14, 14],
      "histogram_bin_count": 32,
      "histogram_sample_count": 80000
    },
    "render_options": {
      "downsample_stride": 1,
      "subpixel_antialiasing": 2
    }
  }
}