Add --preserve-npm

[shadowspawn]

Pull Request Template:

Describe what you did

Added option for preserving npm during node install/activate.

How you did it

Added a new flag and only changed behaviour if flag used. Used tar for the heavy lifting.

How to verify it doesn't effect the functionality of n

Code inspection that this adds a new code path, so existing functionality unaffected.

If this solves an issue, please put issue in PR notes.

#210
#488

If this solves an issue, please include the output of issue that had problems and then the fixed output from the same command.

$ n 8.2.0
$ npm --version
5.3.0
$ npm install -g [email protected]
...
$ npm --version
6.0.0
$ n 8.1.0
$ npm --version
5.0.3
$ npm install -g [email protected]
...
$ npm --version
6.0.0
$ n -p 8.0.0
$ npm --version
6.0.0

Squash any unnecessary commits to keep history clean as possible

Place description for the changelog in PR so we can tally all changes for any future release

[shadowspawn]

Tested on macOS High Sierra, (docker) ubuntu, (docker) gentoo/stage3-amd64.

[shadowspawn]

Could also support n --preserve-npm for using with interactive version selection?

[shadowspawn]

I was looking for issues around specific versions of Linux and found #100 due to /usr/local/share/man being a link on archlinux. Using tar for --preserve-npm extraction overwrites the link with a directory which is undesirable. I will have another go with rsync and see how that goes.