There's no reason we'd want multiple copies of the default settings,
so the defaults can be easily extracted into a static variable to
clean everything up a bit.

Instead, this patch uses Yaml::Null to indicate an empty options
set. Doing things this way cuts down on around 50 lines of code
by keeping most of the searching logic in `yaml-rust`'s domain.

The `value` function computes the value associated with a given key
for a given filename and extension. This was function was liberally
to reduce the repeated logic in the code, and *slightly modifies
behavior*, though in a backwards-compatible way. Some settings were
only customizeable when declared at a global level in the config
file, which may now be specified at the extension/filename level.
This does not (yet) apply to some of the more complicated settings
like `line_length_guide`: see the diff for mor details.

Rather than keeping the theme in a one-off structure entry, index
it with the rest of the preferences values in a Yaml tree.

This relies on a fork of `yaml-rust` which provides mutable acccess.

Utilise `borrowed_or` in my yaml-rust fork to shorten the logic.

Applies to getters:
+ line_comment_prefix
+ syntax_definition_name

This allows for the configuration to specify a line length guide
depending on the file type, for instance, 80 characters everywhere, but
72 in .git/COMMIT_EDITMSG.

Note that this changes the Preferences public API, so there are a fair
few modifications, including to unit tests.

This implements a simple argument parser that isolates 'direct'
arguments (paths, etc) from `--key value` options, and treats the
argument --help specially to display a usage message.

For instance, the invocation `amp --key value src` would launch amp in
the directory src with the option key set to value. Options are not
currently used.

This commit makes it impossible to pass a file beginning with `--` as a
CLI argument: it will be assumed to be an option.

This allows specifying, for instance, `amp <file> --line_length_guide
72` to override any value given in settings.

This is done through a change in Application's constructor, allowing the
provision of overrides. Because of this change, roughly 80-ish tests had
to be modified to pass in empty overrides (thus the long diff).

Cargo.lock had some merge conflicts. This manual merge used cargo to
regenerate Cargo.lock (`rm Cargo.lock && cargo build`).