Add favicon/manifest

---
name: Feature request
about: Suggest an idea for this project

---

## Feature request

**What is the expected behavior?**
Generate different sizes favicons from one source SVG and create a manifest.

**What is motivation or use case for adding/changing the behavior?**
Don't have to manually create and commit different favicon sizes and include them in helmet. I think most people would want to configure a web app manifest as well.

**How should this be implemented in your opinion?**
Use [gatsby-plugin-manifest](https://www.npmjs.com/package/gatsby-plugin-manifest).
I gave it a try and it seemed to work fine just by adding it into plugins. I thought there might be some issues with helmet, but seem ok.

This is the configuration I used. I guess the current coffee cup favicon will need to be recreated as an SVG, unless someone knows the source?
```
{
resolve: `gatsby-plugin-manifest`,
options: {
name: `Kaldi`,
short_name: `Kaldi`,
start_url: `/`,
background_color: `#FFFFFF`,
theme_color: `#FFFFFF`,
icon: `src/img/favicon.svg`, // Need to create this asset
include_favicon: true,
},
},
```

**Are you willing to work on this yourself?**
yes