exec: specify shell to run the command in

[SaschaMann]

Describe the enhancement
Add an option to exec.exec to invoke a specific shell, e.g. /bin/bash, when running the command.
Alternatively, document that exec.exec does not invoke a shell by default.

Code Snippet
Currently:

await exec.exec('/bin/bash', ['-c', `cp -a /Volumes/Julia-*/Julia-*.app/Contents/Resources/julia ${process.env.HOME}`])

Better:

await exec.exec('cp', ['-a', '/Volumes/Julia-*/Julia-*.app/Contents/Resources/julia', `${process.env.HOME}/julia`], {'shell': '/bin/bash'})

Additional information
The docs say that "you can use this package to execute your tools on the command line in a cross platform way". To me that implies it invokes a shell, which it doesn't. This misunderstanding has caused me quite the headache when I tried to find out why cp wasn't working with wildcards.

Disclaimer: I have almost zero experience with Typescript, perhaps this behaviour is obvious to people who regularly use it. 🤷‍♂

[eine]

Ref #232

[alexkappa]

It seems the solution to this issue has been to change the wording in documentation, while the actual issue was not addressed imho.

I'll be submitting a PR which allows defining the shell as with child_process options.