Fix value parser for Option<PathBuf>

[Myriad-Dreamin]

A value parser for OsString cannot parse a command line argument for Option<PathBuf>, which let it failed to specify paths for pack, publish and test commands, which was introduced by 73e059f.

close #1304.

$ ./target/debug/wasm-pack publish abc
thread 'main' panicked at 'Mismatch between definition and access of `path`. Could not downcast to std::path::PathBuf, need to downcast to std::ffi::os_str::OsString
', src/command/mod.rs:74:15

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[drager]

Thank you!