expose some share sub directories

[maxtori]

[lefessan]

I am not completely sure about this PR. In some way, it is in the same spirit as the copy of executables in the root directory, but I am worried that we will have to add particular cases for many things. Maybe we could just copy the _build/install/default/ directory as _install/ so that executables and shared files are available from there ? Anyway, if we keep this PR, here are a few comments:

• I would prefer the work to be done by scripts/copy-bin.sh instead of adding another script
• Since copy-bin.sh mimics a work that is done automatically by drom when make is used instead of drom, the work performed by copy-share-subdirs.sh should also be done automatically by drom in commandBuild.ml (as for executables).
• What if several projects define the same directories in share ?

[maxtori]

It was more for practical use than for a generic use. As it is practical to have the binary of a program copied, it is sometime useful to have some subdirectories of share copied like www for js projects. I think copying the whole _build/install/default would be a bit overkill. I'll look into commandBuild.ml and maybe put it as an option. I think the share directories are package separated so I don't think there can be any conflict.