Merge shell in main build?

[francisbouvier]

shell usage is for now a separate build and a separate CLI browsercore-shell.

We could merge it in the main build and use it as a optional argument, ie.

• browsercore <arg> to launch the main server
• browsercore --shell <arg> to launch the shell

It makes more sense as a CLI usage perspective (only 1 binary) and simplify the build process.

The only downside is that it adds linenoise as a build requirement, add therefore increase the binary size. But I don't think it's a big problem, as linenoise is meant to be small.
We can also add a build option --no-shell (non-default) to keep the option to build from sources linenoise dependancy and without shell support.