Add button content prop

**Is your feature request related to a problem? Please describe.**
Currently it is not possible to change the button text.

**Describe the solution you'd like**
It should be possible to pass a `label` prop.

**Additional context**
![image](https://user-images.githubusercontent.com/21159914/93003820-32f84700-f542-11ea-8974-2ae06d336172.png)

