Upkit

Improvements:
* `content` and `exclude` only select files and folders in `source` instead of anywhere.
* Add documentation.

This release bundles quite a lot of useful features. Noticeably, Upkit projects now can resolves Nuget and `git` dependencies without a separating step.

New features
* Add `nuget:` scheme to `source` attribute. For example, declaring `source: 'nuget:NewtonSoft.Json11.0.2` will download the package automatically and link to the project.
* Add `git:` scheme to `source` attribute. For example, declaring `source: 'git:https://github.org/some-repo' will clone the repository and link to the project automatically.
* New `create-package` command, to quickly create a reusable Unity (re)package
* Add `exclude` attribute to Linkspec to exclude unwanted data.
* `link` command now use `upkit.yaml` as default config file name. Linking a project can be as simple as `upkit link`.

Improvements
* Better error when missing `package_folder`.
* Relax the need for content path when resolving nuget or git packages. The tool looks for `linkspec.yaml' in both package root and its sub `content` folder.
* Overwrite links for all cases, not just existing links.
* Linkspec overwriting does not require `linkspec` property. When there is either `content`, `links`, or `external_links` defined in a `link`, the link will overwrite the package's predefined linkspec.
* `link` command will create Unity project folder, including `Assets` and `Plugins` instead of using links.
* `-c` parameter of `link` command now becomes positional.
* `__projects__`, `__assets__` and `__plugins__` are builtin parameters when linking projects.
* `create-package` template now has default support for `Packages` folder in Unity 2018.

Bug fixes:
* 2: issue with `git:git`

* Update README.md
* Change project name to `upkit`.
* Fix 1

Fixes
* Tool failed to parse command line on Python 3.3 or above due to parse_args() does not raise exception.

New features
* Remove the required `target` property in each link, when sub-links in linkspec is available. For example, given `A` is a package having two links in its linkspecs, it could be declared as following in the link config:
Yaml
config.yaml
links:
- source: '{{__dir__}}/A'

instead of
Yaml
config.yaml
links:
- source: '{{__dir__}}/A'
target: '{{__dir__}}/B'