Allow reuse of existing node_modules

[sewe]

Hi,

I have the following use case: I’m using gitbook-cli to generate documentation as part of an automated build. For every build, my CI server does a fresh checkout of the book’s sources and removes any generated files (git clean -df). This includes the node_module directory created by gitbook install; thus, the CI server needs to re-download any required plugins during each build.

Obviously, this is undesirable. Would it hence be possible to point gitbook to a different (global) node_module directory that lives outside of my checkout directory and can be reused across builds? (The only workaround I can think of is checking in node_modules, so that it survives the git clean -df, but that’s far from ideal.)