Possibility to indicate the absolute path through --installation_path on all OSs in a homogeneous way

[bdovaz]

Issue Checklist

• [x] I'm using the latest u3d version
• [x] I ran u3d --help
• [x] I read the README
• [x] I made sure that a similar issue doesn't exit

Issue Description

I would like to be able to install the editors by naming the directories homogeneously in all the OSes.

Right now with the "--installation_path" parameter:

• Windows: the absolute path can be specified.
• macOS: only the path to the volume can be specified.
• Linux: I haven't used it so I don't know.

In macOS I would like to be able to indicate the path to the directory where it will be installed. Right now looking at the next line of code you can see how it is hardcoded with the prefix "Unity_":

https://github.com/DragonBox/u3d/blob/a46fc319139bd6ebe983b4e002bd3f22a85120cd/lib/u3d/installer.rb#L215

Can it be modified to allow the relative path to be indicated from the volume? Thank you.