Support for ECDSA

[kilianmh]

I would like to use e.g. ES256 in some applications. See fukamachi/jose#3. For Jose to support this we have to update pem first. Before sending a PR i wanted to ask how the implementation should be structured:

• Should we integrate ECDSA in read-from-file? This works simple with private key files but a bit more tricky with public key files.
• Or would you prefer to have new exported function such as read-ec-from-file?

[fukamachi]

Although I'm not sure how it would be tricky, it may be better to have different functions for each algorithm if they require different sets of arguments, like read-rsa-public-key and read-ecdsa-public-key.