feat(theme): Branded theming via brand.yml

[gadenbuie]

Adds support for branded theming via bs_theme(brand = TRUE).

There are few choices:

# Automatically find `_brand.yml` in app dir or its parents
bs_theme()

# Find `_brand.yml` in app dir or its parents, or fail otherwise
bs_theme(brand = TRUE)

# Direct path to a brand.yml file
bs_theme(brand = "path/to/brand.yml")

# Ignore an existing `_brand.yml` file, apply no branding
bs_theme(brand = FALSE)

# Provide a brand definition inline
bs_theme(brand = list(color = list(primary = "#466683")))

Or in YAML where bs_theme() is called:

theme:
  brand: true

theme:
  brand: false

theme:
  brand: "path/to/brand.yml"

theme:
  brand:
    color:
      primary: "#466683"

TODO

• Tests
• Documentation - Update function docs in bs_theme()
• Documentation - Article on pkgdown site?
• Build ignore Monda.ttf and download it as needed on app load

[gadenbuie]

We'll need to update this link before release (remove dev from URL)