text: Title: Disable `DiscreteNorm` by default for `imshow` plots?
  Body: <!-- Thanks for helping us make proplot a better package! If this is a bug report, please use the template provided below. If this is a feature request, you can delete the template text (just try to be descriptive with your request). -->
  
  ### Description
  
  In all the examples available for this package and in my tests, the colorbar maximum number of bins is fixed to 10. There is doesn't seem to by any to changing this
  
  ### Steps to reproduce
  
  A "[Minimal, Complete and Verifiable Example](http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports)" will make it much easier for maintainers to help you.
  
  ```python
  fig, ax = proplot.subplots()
  pc = plt.imshow(p.image, extent=[file.bounds[0], file.bounds[1], file.bounds[2], file.bounds[3]], cmap=my_cmap, vmin=0.5, vmax=1.5)
  plt.colorbar(pc, ax=ax)
  ax.set_xlim([0, 1])
  ax.set_ylim([0, 1])
  ax.set_xlabel(r"$x/R$")
  ax.set_ylabel(r"$y/R$")
  plt.show()
  ```
  
  **Actual behavior**: [What actually happened]
  ![image](https://user-images.githubusercontent.com/11050889/102356830-fe1ae680-3fad-11eb-925b-372c5188970f.png)
  
  
  ### Equivalent steps in matplotlib
  
  Please make sure this bug is related to a specific proplot feature. If you're not sure, try to replicate it with the [native matplotlib API](https://matplotlib.org/3.1.1/api/index.html). Matplotlib bugs belong on the [matplotlib github page](https://github.com/matplotlib/matplotlib).
  
  ```python
  fig, ax = plt.subplots()
  pc = plt.imshow(p.image, extent=[file.bounds[0], file.bounds[1], file.bounds[2], file.bounds[3]], cmap=my_cmap, vmin=0.5, vmax=1.5)
  plt.colorbar(pc, ax=ax)
  ax.set_xlim([0, 1])
  ax.set_ylim([0, 1])
  ax.set_xlabel(r"$x/R$")
  ax.set_ylabel(r"$y/R$")
  plt.show()
  ```
  
  ![image](https://user-images.githubusercontent.com/11050889/102357071-40dcbe80-3fae-11eb-8919-06f7c30fed9d.png)
  
  