Allow running fpm from out-of-package directory

[milancurcic]

In other words, allow specifying a package directory to build, install, test, and run commands.

Cargo has cargo build --manifest-path=dir/Cargo.toml.

This would be useful in automating fpm batch jobs. For example, in combination with #410, this

fpm run --manifest-path=/path/to/my/fpm/package/fpm.toml --directory=/path/to/my/run/dir

would run an executable from a package in /path/to/my/fpm/package in a directory /path/to/my/run/dir.