Add is-installed-globally for auto-detection

[timdeschryver]

See #112 (and avajs/ava#1407)

[nexdrew]

These changes LGTM, assuming we consider is-installed-globally@0.1.0 (and global-dirs@0.1.0) to be stable. Based on my testing with Node 8/npm 5 and this commit, I'm not sure we're ready to do that.

@sindresorhus Perhaps you can elaborate a bit (or point us to an issue) on the current problem with npm 5? Looks like npm 5 symlinks "global installs of a local package" into the global root (i.e. result of npm root --global) instead of copying the package there (as it did in npm 4)?

[sindresorhus]

assuming we consider is-installed-globally@0.1.0 (and global-dirs@0.1.0) to be stable.

It works from my local testing and the tests, but there might be edge-cases I haven't considered. I wouldn't say it's that important though. Worst case, it shows the wrong install flag and we fix it. It already shows the wrong flag when installed locally, so it's not like it would be worse. Happy to keep this open for a little bit so people can try it out though.

Looks like npm 5 symlinks "global installs of a local package" into the global root (i.e. result of npm root --global) instead of copying the package there (as it did in npm 4)?

Yes, I have a todo to open an issue on the npm issue tracker about it.

[sindresorhus]

@tdeschryver Can you fix the merge conflict?

[timdeschryver]

No problemo!

[sindresorhus]

Thanks :)