Add textarea styling. Allow to set border for all input types

1. Allow users to overwrite border for all inputs by overwriting border variables.
2. Add styles for textarea

Before

![before](https://s3.amazonaws.com/upload.screenshot.co/1191d6cdba)

After set the `$picnic-border-weight: 3px` and `$picnic-border-color: orange`

![after](https://s3.amazonaws.com/upload.screenshot.co/1552aca899)