Support --invoke option for emscripten files without _start function

[fschutt]

When running emscripten files, the CLI now supports the --invoke option to invoke a function by name.

Review

• Add a short description of the change to the CHANGELOG.md file

[epilys]

@fschutt btw can you share the emscripten files you used for testing?

[fschutt]

@epilys I tested it with the python-0.2.0.tar.gz

unpack python-0.0.2.tar.gz
wasmer run ./python-0.0.2.tar.gz/bin/python37.wasm
wasmer run --invoke _main ./python-0.0.2.tar.gz/bin/python37.wasm

python-0.0.2.tar.gz

I tested manually that it works (after all, it's just a single variable difference to the run() code). I'd want to avoid committing large files to git if possible.

[epilys]

bors r+

[bors]

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