r5

* fix the make_dirs custom filter to behave as specified. Previously it would
  create a directory from it's piped input, instead of it's parameter as it was
  supposed to.

r4

This release marks the first real brand new feature: side-effectful filters.
These are custom `pongo2` filter extensions that make it more useful as a
systems templating utility by allowing single templates to act a little more
like scripts, removing the need for bash-boilerplate in complex templating
scenarios.

* Add the first custom scripting filters to p2
  * Add `--enable-filters` and `--enable-noop-filters` command line arguments.
* Add `write_file` and `make_dirs` side effectful filters.

Release 3

* Switch to upstream master of pongo2 (notably adds the `split` filter)
* Add --output command.
* Properly handle shell-quoting in shell-style environment files.

Merge pull request wrouesnel#2 from wrouesnel/update

Update dependencies, lock to pongo.v3

travis take 5.

First release build. These exist to make building Dockerfiles easier by

providing downloadable binaries.