Include a base directory "icestudio" in the zip files

[javacasm]

When the release zip files is unzip all the files go the current directory.
In my opinion, it would be better include a base "icestudio" directory.

[Jesus89]

Hi,

I think in Windows 10 you can select the option (Create a directory and unpack) to avoid this problem. Also in Linux the directory is automatic created if needed.

However, I think it is a good proposal in order to create a common multiplatform behavior.