Add delete support

[jayvdb]

As a repository manager, renaming files is a fairly common requirement.

Currently the name of a target file can be changed, but the old filename isnt scheduled for deletion at the same time.

It may seem obvious that the old file needs to be deleted, especially when the template writer and template users (repo managers) are the same people, but newbies dont learn 'obvious' without clues, and a central configuration management should be able to automatically manage updated to a large number of managed projects which have different teams/repo managers.

The new-file template rule and the old-file delete rule should be together in the same section of the mobanfile. i.e. under targets:

Having learnt from copy: which I think should be a plugin per #165 , I believe delete should also be a plugin. It should be a core plugin, as it satisfies a core requirement of moban. It really should be a plugin, because people should be able to customise moban with plugins which can perform deletions in a way that the moban engine understands.

The easiest syntax would be:

targets:
  - foo:  # implicit empty/null/etc

However that allows unintended deletes to occur very easily.

Much better would be to require an explicit datatype, such as false

targets:
  - foo: false  # explicit delete

If yaml datatype extensions in #165 are implemented, the following would also be possible

targets:
  - !delete foo  # explicit delete

[jayvdb]

Another approach is to handle renames more natively, such as:

targets:
  - output: foo
    template: bar/foo.jj2
    removed_output_names: baz  # will be deleted if present

A real delete could then be still achieved as

targets:
  - output:  # doesnt matter that this is empty/false/etc, as this key is only to ensure datatype is mapping
    removed_output_names: baz

Then other types of deletes can be designed when requirements are clearly defined and the long multi-line syntax is too cumbersome.

[jayvdb]

Moved to milestone 0.5 , so we have the 'copy' changes released before starting on delete.