GeezerVison

feat: Add more tags

feat: Add `linkedin` icon

Closes feathericons#77

feat: Add more tags to tags.json

ci: Update .travis.yml

docs: Update README.md

feat: Update API

BREAKING CHANGE: 

Each icon in the `feather.icons` object is now an `Icon` object with a `name`, `contents`, `tags` and `attrs` property.
```js
/* BEFORE */
feather.icons.x
// '<line ... /><line ... />'

/* AFTER */
feather.icons.x
// {
//    name: 'x',
//    contents: '<line ... /><line ... />`,
//    tags: ['cancel', 'close', 'delete', 'remove'],
//    attrs: {
//      class: 'feather feather-x',
//      xmlns: 'http://www.w3.org/2000/svg',
//      width: 24,
//      height: 24,
//      viewBox: '0 0 24 24',
//      fill: 'none',
//      stroke: 'currentColor',
//      'stroke-width': 2,
//      'stroke-linecap': 'round',
//      'stroke-linejoin': 'round',
//    }
// }
```

`feather.toSvg()` has been deprecated in favor of `feather.icons[name].toSvg()`:
```js
/* BEFORE */
feather.toSvg('x')

/* AFTER */
feather.icons.x.toSvg()
```

`feather.replace()` now copies all attributes on the placeholder element (i.e. `<i>`) to the `<svg>` tag instead of just `class` and `id`:

```html
<i data-feather="circle" id="my-circle" class="foo bar" stroke-width="1"></i>
<!--
<i> will be replaced with:
<svg id="my-circle" class="feather feather-circle foo bar" xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="10"></circle></svg>
-->
```

feat: Updated replace() to pass id from placeholder element (featheri…

…cons#193)

fix: Fix adding unwanted class 'null'

fix: Fix `bold` icon

feat: Add `underline` icon