Map the shell key to the shell option of spawn

The command on a run entry is executed using Node.js' spawn command. The spawn command can be configured to run the command in a shell. By default, this is only enabled on Windows. Consider allowing the user to control this option using the shell key on a run entry.

When the shell option is enabled, the command is run through the default shell. This is consistent with how commands are run in a CI workflow. One benefit of doing so is that environment variables in the command string are expanded. One drawback is that more care must be taken with quoting.

An alternate approach is to expand environment variables in the command when shell: true is set, but don't actually attempt to run the command in a shell. More research may be needed to determine what level of control to give to the user.