- Add: `TextImage._get_render_data()`, implementing image quantization
  for text-based styles towards indexed-color rendering support.
- Add: `TextImage._XTERM_240_PALETTE_IMAGE`.

- Add: `DIRECT` and `INDEXED` render methods.
- Add: *method* style parameter and format spec field.
- Add: Attributes:

  - `_FORMAT_SPEC`
  - `_render_methods`
  - `_default_render_method`
  - `_render_method`
  - `_style_args`

- Add: `._check_style_format_spec()`.
- Add: *method* parameter to `._render_image()`.
- Change: Update class docstring.

- Add: Use the *method* parameter of `._render_image()`.
- Add: Implement the `INDEXED` render method.

- Fix: Typo in `._get_render_data()`.

- Add: Mention indexed-color support in the requirements.
- Change: Update the list of supported terminal emulators.

- Fix: Use the non-standard semi-colon-delimited indexed-color SGR
  control sequences, as they're more widely supported than the standard
  colon-delimited counterparts.

Refs: #90 (comment)