Add support for importing keys from a file. Fixes #75

It is common for people to send their keys as attachments in emails. Currently it's a bit cumbersome to import them in Mailvelope, since you need to open the key in an external editor, copy the text, and then paste it into Mailvelope. Additionally, if you have some key helper application installed, you have to make sure you open the key in a text editor and not the helper application (which is usually the default option).

Ideally there should be some method to also just paste in a URL to a key since it is quite common for people to store public keys online. However, it might make the UI a bit more messy, so I'm not sure if it's worth it.