This PR adds support for extracting metadata from `<form>` elements.
This was asked #153 and came up elsewhere too, I believe. You can use
it like this:
```html
<form class="MyForm" action="/">
<input type="file" />
<input type="hidden" name="bla" value="12333">
<input type="text" name="yo" value="1">
<button type="submit">Upload</button>
</form>
```
```js
uppy.use(DragDrop, {
target: '.MyForm',
locale: {
strings: {chooseFile: 'Выберите файл'}
},
setMetaFromTargetForm: true
})
```
Then UI acquire type plugins, like Dashboard, FileInput and DragDrop,
before mounting themselves or doing anything else, extract FormData
from the `target` `<form>` element (it must be a form currently), and
merge the object with the global `state.meta`. This is done via
`setMeta` method on core. Then, when a file is added, `state.meta` is
merged to that file’s meta right away. The ability to add more fields
via UI plugin MetaData is also still there, though probably needs an
update.
In addition a new `meta` option is added in core:
```js
const uppy = Uppy({
debug: true,
autoProceed: true,
meta: {
username: 'Artur'
}
})
```
This way some data can be set right away, it becomes the initial
`state.meta` object.
