This PR adds functionality for users to specify which `icon_pack` to use (`fas` or `fab`; see: https://fontawesome.com/how-to-use/on-the-web/setup/upgrading-from-version-4) in the `config.toml` file, so that users can use the solid icons as well as the brand icons.
```
[[params.social]]
url = "https://twitter.com/"
icon = "twitter" # icon name without the 'fa-'
icon_pack = "fab"
[[params.social]]
url = "https://facebook.com/"
icon = "facebook-f" # icon name without the 'fa-'
icon_pack = "fab"
[[params.social]]
url = "https://linkedin.com/"
icon = "linkedin-in" # icon name without the 'fa-'
icon_pack = "fab"
[[params.social]]
url = "mailto:youremail@email.com" # For a direct email link, use "mailto:test@example.org".
icon = "paper-plane" # icon name without the 'fa-'
icon_pack = "fas"
```
