Add --target-dir parameter to get-app

[pgreenwo]

Issue: Feature Request

Is your feature request related to a problem? Please describe.

When installing the Raven chat app with bench, I discovered that my install was failing because of the capitalisation in the Raven repo name, which was used for the target directory name. The yarn install process looked for an uncapitalised directory name.

A git clone CLI command would have allowed me to fix this in the CLI instruction by specifying the target, but using bench meant that this option wasn't available.

Describe the solution you'd like

Be able to do this:

bench get-app raven https://github.com/The-Commit-Company/Raven.git --target-dir /workspace/development/frappe-bench/apps/raven

Describe alternatives you've considered

1. Forking the repo and renaming it to lower case.
2. Forking the repo and adjusting the installation location parameters.
3. Modifying yarn to make it case insensitive.

Additional context