Allow the setting of an absolute path for IO

[mtwilliams5]

Enforcing that all input passed to parameters like -in and -out be prefixed with openssl/ is a bit restrictive, particularly for -in, as the intention there is to be able to specify an existing source file that you have for conversion.

This PR allows the IO parameters to be set as an absolute path. If the value starts with either / or ~/, it will not add openssl/ to the start of it; if it starts with any other character, then the old behaviour prevails.