Allow the preview stylesheet to be overridden for accessibility

Read an override.css stylesheet when the app loads in order to allow those with poor eyesight to read notes in the app. In recent versions lists and pres and heading-separated sections have become difficult to read or discern. Zooming in isn't enough to make the content legible.

#### Steps to reproduce
1. Have poor eyesight
2. Open the app
3. Create notes
4. Try to read them

#### What I expected

Most of the app is very legible (both the layout and text content) however some types of content are difficult to read.

#### What happened instead

In markdown mode
* Can't zoom in
* The font family can't be changed
* Some text is serif and other text is sans-serif
* h2s are gigantic, so when zooming becomes possible again they will take up most of the screen after zooming in to see regular text
* pre text color is too light to read

In edit mode (not sure if the layout is controlled with css though)
* Unordered list line spacing is too compact
* Unordered lists do not have hanging indents
* There is not enough margin underneath h1 and above h2 elements to visibly separate sections of the note

#### Simplenote version

Simplenote 1.4.0

#### macOS version

macOS 10.14.2

Start at the app-load path and inspect how stylesheets are applied in markdown mode and edit mode. Verify where an override.css file could be read, then confirm that custom styling can improve list spacing, heading separation, font choices, zoom behavior, and preformatted-text contrast without breaking the existing layout.

Written by the indexing model from the issue text.