Allow multiple compile_commands.json

[Timmmm]

I'm using the VSCode extension, which allows you to specify a directory for compile_commands.json (good, I don't want to pollute my source with build files). However I'm using a workspace with several separate projects (which depend on each other). Each has its own compile_commands.json.

This is the same situation as #835 - he is using a hacky sed script to concatenate the compile_commands.json into one file. That's not a very good solution though because:

a) I have to make my build system know about all of the compile_commands.json files and rebuild the combined one when any of them changes. It's not going to be pretty.
b) #835

Instead it would be better if you could just give a list of compile_commands.json files to cquery and it would monitor all of them.

[Timmmm]

I did find that in addition to the compilationDatabaseDirectory option there is compilationDatabaseCommand which would allow me to combine the multiple compile_commands.json. It isn't exposed in the VSCode extension but it looks like adding it would be easy.

However a command is not quite as good because you can't monitor it for changes. On the other hand, I had a look through the cquery source code and I can't see if it actually monitors compile_commands.json for changes anyway so maybe it is just as good.