protoc arguments not quoted

[VolkerKamin]

Issue:
If --proto_path contains blanks, the missing quotation causes "no such file or directory" error.

Example:
[command] D:\protobuf\bin\protoc.exe --proto_path=D:\workspaces\git\sdhapp\Stammtisch der Helden\src --proto_path=D:\protobuf\include --java_out=D:\workspaces\git\sdhapp\Stammtisch der Helden\src-gen D:\workspaces\git\sdhapp\Stammtisch der Helden\src\de\volkerkamin\sdh\db\sdh.proto
[protoc] D:\workspaces\git\sdhapp\Stammtisch: warning: directory does not exist.
[protoc] der: No such file or directory

Platform:
Windows 10
Eclipse Photon (Build id: 20180619-1200)
Protocol Buffer Editor (2.3.2.201609161849)
protobuf-3.6.1