Allow pre-release for specific packages only

[jaroel]

I'd like to be able to install one specific package with a pre-release, but not the rest.
I can currently set allow_prereleases = true, but I'll have to pin every other package to a stable version range.

I'd rather have pipenv to prefer stable releases, but allow me to set any version for a package.

For example, I'd like to use the black package with version 18.6b2.
I do not want the pre-releases for pylint-django, or any other dependency as they do not work at the moment.

This one doesn't work, but I'd like it to :)

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]

[dev-packages]
black = "18.6b2"
pylint-django = "*"

[requires]
python_version = "3.6"

[pipenv]
allow_prereleases = false

This one does work. Note the pylint and astroid pinning.

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]

[dev-packages]
pylint = "==1.*"
black = "*"
pylint-django = "*"
mypy = "*"
astroid = "==1.*"

[requires]
python_version = "3.6"

[pipenv]
allow_prereleases = true

[gsemet]

Similar to #1760.

[uranusjr]

Closing this to centralise the discussion.