Consider a --allow-dirty option for mlar commands

[commial]

Now that the FailSafe reading is quick enough (see #157), it could be nice to have a --allow-dirty flag for mlar command such as cat and extract, to allow them to read from:

• a broken archive
• a non-seekable input (STDIN, network socket, etc.)

Without the need to repair it first.

As the FailSafe reading offers less guaranties than the normal read (properly made archive, encryption tag checks, etc.), allowing a read from them should be an explicit choice, for instance through a --allow-dirty flag.