Version 1.1.0: Priority forwarding & various improvements

This release (originally released 2018-10-09) of journaldriver includes the following features and fixes:

• journaldriver now correctly recognises and forwards journald's log priorities. Thanks to @ttencate for suggesting and implementing this feature!

• journaldriver now deals with potential issues while writing the cursor file better than in the previous version. See issue #2 for more information.

• The previous CURSOR_POSITION_FILE environment variable has been replaced with CURSOR_POSITION_DIR, which is expected to point at a writable directory in which cursor positions are persisted.

  This option was previously undocumented and is not used by the NixOS module. The defaults have not changed, so in setups that previously did not customize this option nothing needs to be done.

• journaldriver depends on significantly fewer Rust libraries now due to a move to a more lightweight HTTP client (ureq)

In order to make use of journald-priorities applications submitting messages into the journal must add severity information. The easiest way to do this is to use priority prefixes. Please consult the README for additional information.

In other news, journaldriver is now included in NixOS 18.09 and can be enabled via services.journaldriver.enable = true;. Configuration options for running on non-GCP machines are provided, please consult the NixOS module documentation for those.