feat: Open application in a repository

[siketyan]

Closes #2

After cloned a repository, we often want to open some application such as IDE or editor in the cloned repository directory.
We utilised that in a simple way.

To use this feature, you need to configure applications you want to open first.
The following example shows an application entry opening Visual Studio Code.

[applications.vscode]
cmd = "code"
args = ["%p"]

Note that an argument %p is replaced by the actual repository path.

Now you can use ghr clone --open or ghr open:

ghr clone siketyan/ghr --open vscode
ghr open siketyan/ghr vscode