Add augeas() function

This PR adds a new `augeas()` function, which does on the master side pretty much what the `augeas` type does on the client side.


*Example:*

    augeas("proc        /proc   proc    nodev,noexec,nosuid     0       0\n", 'Fstab.lns', ['rm ./1/opt[3]'])

Would result in:

    "proc        /proc   proc    nodev,noexec     0       0\n"

- *Type*: rvalue