enh: save buffer marks between runs

[rodrigolive]

Summary

Fix marks buffer marks being lost after Prettier runs.

Fixes this issue #251

Test Plan

• add a mark with ma, move around, jump to it with 'a
• "uglify" a line in your code, run :Prettier to fix it
• now do 'a, vim should jump the cursor to the marked position with ma. The line might not correspond exactly with the previous line due to lines changed by Prettier, but it should be close.

[mitermayer]

Thank you