install-path customization

[Gankra]

As noted in #41, the shell/powershell installers unconditionally install into cargo home, but we should let the user configure that.

This should possibly support some amount of templating, as seen in the config for cargo-binstall.

Theoretical usecases with strawman syntax:

• install-path = "{cargo}" (use cargo's global install dir)
• install-path = "{npm}" (use npm's global install dir?)
• install-path = "~/.my-app/" (install to a hardcoded dir in the user's home)
• install-path = "{xdg}/my-app/" (install to whatever the hell xdg says you're supposed to do)