[performance] Removing dependencies / splitting the package

[aminya]

A follow up to #277 to further improve the performance.

As I mentioned in https://github.com/JunoLab/atom-julia-client/issues/744, it is best if we can use X-terminal package as the REPL. That increases the modularity, and finally, it increases the performance.

We can also start moving some parts of the code to the atom-ide-community to allow other languages to use the same functionality.

• [x] Move Katex -> #277 and https://github.com/JunoLab/atom-julia-client/pull/750
• [ ] Use X-terminal package

Here is an analysis of the loading time caused by xterm dependencies: Link