Make `level` dependency installation optional

First thanks for this module, I use it a lot and it’s great work!

Is your feature request related to a problem? Please describe. level uses leveldown which is a native module. Installing this native module is slow and can more than double the time required by npm install on a project. Considering the code provides alternative stores such as files and in-memory, there should be a way to opt-out of installing this native module.

I am not 100% sure but I am assuming stores are used to cache results between different runs (correct me if I’m wrong). If so I think faster read/writes or smaller cache size are nice-to-have, but I am not sure this justifies doubling the time required by npm install.

Describe the solution you’d like Any solution works, but one that might work would be:

• try to require('level'). If this throws, use require('level-js') instead
• replace level by level-js in package.json dependencies

I.e. to use the native module, level must be manually installed by users.

Describe alternatives you’ve considered optionalDependencies do not work here because they are still installed and are meant for a different purpose.

[kucherenko]

It makes sense to make separate levelDB package for checking big repos. I think I will do that

[ehmicky]

Just for information purpose: I’ve tried each store on a medium-size repository and got the following average times: 20 seconds for level and 15 seconds for both memory and files.

Another perk for memory is that it does not create .jscpd folder, although that’s not a huge issue (just need to add it to .gitignore).