Codeblocks now have line numbers, highlighting, bg override

[d0c-s4vage]

Adds the ability to enable line numbers in code blocks and highlight lines

Specifically:

• Curly brace attributes on code blocks are now supported
• the theme can now be set in the YAML metadata:

  ---
  theme: light
  ---

• a new styles.code YAML metadata value can be used:

  ---
  styles:
  code:
    style: zenburn
    inline_lang: python
    bg_override: "#aabbcc"
  ---

• the --style CLI argument is removed (should be replaced by the feature described in #190)

See #164 See #89

[d0c-s4vage]

From this markdown:

---
theme: dark
styles:
  slides:
    fg: "#c0c08f"
  code:
    style: monokai
    inline_lang: python
    bg_override: "#302030"
---

The `a = 100 * "hello"` metadata that can be prefixed in slides includes these top level
fields:

```python {.numberLines startFrom="1" hllines=1,5,7-9 .nobackground}
def hello():
    """Text
    """
    # comment
    print("Hello world")
    print("Hello world")
    a = 1 * 20 + 30
    X = 1 * 20 + 30
    y = 1 * 20 + 30
    a = 1 * 20 + 30
    k = 1 * 20 + 30
```

You get this:

[d0c-s4vage]

Note: this PR includes breaking changes to how syntax highlighting styles are defined:

• the --style CLI argument is removed (should be replaced by the feature described in #190)
• the theme can now be set in the YAML metadata
• a new styles.code YAML metadata value can be used:

styles:
  code:
    style: zenburn
    inline_lang: python
    bg_override: "#aabbcc"