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

[cpsievert]

Should we be throwing an error if version is less than 5?

[gadenbuie]

I'm not sure... we don't have a fully adapted branded theme for BS <= 4 but there are still a number of features of brand that are useful. It's also possible that a user could fill in their own implementation for BS <= 4 using only the tools already present in brand.yml. So erroring here feels too aggressive. I'll add a warning instead.

[cpsievert]

Looks great overall! Consider this approved when all suggestions are considered/addressed and if there's no new major code changes. I will be mostly away until the 10th, but if you have questions that can be addressed asynchronously, I'll be checking Slack/GitHub periodically.