Extend key -read and -write functions

[chris2511]

If an existing key or the new key is not intended to be stored in a file, the current implementation requires the creation of temporary files with all its odds (cleanup, symlink attack, mktemp)

This change allows to provide the key in many different ways:

A filename of "-" translates to for reading and for writing.

If the file named by "filename" does not exist, check whether it contains a "-----BEGIN" and try interpreting the filename itself as key.

Extend documentation in README.md