Add APPIMAGE_COMP env var

Fixes #23.

Author: TheAssassin

README.md

@@ -39,3 +39,4 @@ linuxdeploy-plugin-appimage can be configured using environment variables.
 - `VERBOSE=1`: set this variable to any value to enable verbose output
 - `OUTPUT=filename`: change filename of resulting AppImage
 - `NO_APPSTREAM=1`: skip checking AppStream metadata for issues
+- `APPIMAGE_COMP=...`: compression algorithm appimagetool/mksquashfs should use (e.g., `xz`, `gzip`), see [appimagetool's repository](https://github.com/AppImage/AppImageKit/) for more information

src/main.cpp

@@ -139,6 +139,12 @@ int main(const int argc, const char* const* const argv) {
         args.push_back(strdup("--no-appstream"));
     }
 
+    auto comp = getenv("APPIMAGE_COMP");
+    if (comp != nullptr) {
+        args.push_back(strdup("--comp"));
+        args.push_back(strdup(comp));
+    }
+
     args.push_back(nullptr);
 
     std::cerr << "Running command: " << pathToAppimagetool;