* Support dark mode theme

* Add dark mode to 404 and package pages

* Fix tests

Adds support to other VCS systems as per the specification.
Defaults to `git` for backwards compatibility.
Renames the internally used `.GitURL` to `.RepoURL` for clarity.

<details>
<summary>Background</summary>

Sally renders two kinds of pages:

- packages: These are for packages defined in sally.yaml
  and any route under the package path.
- indexes: These list available packages.

The latter--indexes was previously only supported at '/', the root page.

This leads to a slight UX issue:
if you have a package with a / in its name (e.g. net/metrics):

- example.com/net/metrics gives you the package page
- example.com/ lists net/metrics
- However, example.com/net fails with a 404

</details>

This adds support for index pages on all parents of package pages.
Therefore, if example.com/net/metrics exists,
example.com/net will list all packages defined under that path.

Resolves #31

* update: removing redundant go-source tag

* fix: removing /.idea and adding dir to gitignore

* fix: removing /.idea and removing branch field from from templateData

* fix: pkgHndler defaultBranch rem revert

* fix: minor typo

* fix: removing assertion of template responses

* fix: tabs

* Update handler_test.go

* Update handler_test.go

* Update handler_test.go

---------

Co-authored-by: Anuragkillswitch <70265851+Anuragkillswitch@users.noreply.github.com>

Support specifying the branch from which source will be served in
the YAML configuration.