[WIP] Added --save option to install command

[blarghmatey]

I added a pair of new command options to the install command. --save
will append the specified packages to the end of the 'requirements.txt'
file. --save-dest will take a filename or path argument and the
specified packages will be appended to the given file.

I am opening this pull request to see if there is any interest in adding this to pip and if so to gather any feedback about things that I may be doing wrong.

[piotr-dobrogost]

For information, this feature was requested in issue #1479

[blarghmatey]

Is this a desired PR? If so, are there any glaring issues with it? I'm happy to address any shortcomings. Also, advice on the best file/location to add appropriate tests would be appreciated. Trying to navigate the test structure left me rather disoriented.

[noelvo]

This PR is great. I hope it will get merged soon. I have tested out and it works just fine. --save option will save developers a lot of time.

[yassineAlouini]

Great one. Hopefully, this will get merged soon 👍

[fitoria]

+1

[bitzip]

+1

[dstufft]

Accidentally closed this, reopening. Sorry!

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow pip has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master. Unfortunately, this pull request does not cleanly merge against the current master branch.

If you do nothing, this Pull Request will be automatically closed by @BrownTruck since it cannot be merged.

If this pull request is still valid, please rebase it against master (or merge master into it) and resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to rebase/merge and resubmit this Pull Request, here is an example message that you can copy and paste:

I added a pair of new command options to the install command. --save
will append the specified packages to the end of the 'requirements.txt'
file. --save-dest will take a filename or path argument and the
specified packages will be appended to the given file.

I am opening this pull request to see if there is any interest in adding this to pip and if so to gather any feedback about things that I may be doing wrong.

---

*This was migrated from pypa/pip#2988 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

[BrownTruck]

This Pull Request was closed because it cannot be automatically reparented to the master branch and it appears to have bit rotted.

Please feel free to re-open it or re-submit it if it is still valid and you have rebased it onto master or merged master into it.