Refactor PdftkRunner to reusable ShellRunner

This also moved escaping completely into ShellRUnner, so it now requires
args to be an array.
PDF decryption support with qpdf

This allows to handle owner password protected PDFs automatically and
user password protected PDFs by specifying the password.
Merge branch 'qpdf-decryption-support'
Bump version to 0.2.0