[performance] Removing dependencies / splitting the package

[aminya]

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

As I mentioned in JunoLab/atom-julia-client#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.

• Move Katex -> Rollup #277 and Katex atom-julia-client#750
• Use X-terminal package

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