Support passing --index-url to pip install command

[DanCardin]

In writing this, it turns out I can do pipx install --spec https://custompypi.com/api/pypi/pypi/packages/examplepackage/1.0.0 binary, but it'd be nice if the interface was more normal to how installation would typically work

pipx install --index-url https://custompypi.com/api/pypi/pypi/simple binary
pipx install --index-url https://custompypi.com/api/pypi/pypi/simple --spec examplepackage==1.0.0 binary

[cs01]

Nice, I can add this.

[DanCardin]

great! it occurs to me it'd be useful on upgrade as well (unless upgrade will remember where it got the original install from)

[cs01]

This has been added in pipx-app 0.10.4.0.