add formatOnSave setting

[Andriamanitra]

closes #71

TODO:

• Create a method for formatting request that both preSave and formatAction can use
• Deserializing null in json.lua should give json.null instead of nil #66 causes "[µlsp] No results" to be logged when saving a file that doesn't require any formatting, so that should be fixed before this can be merged
• Test with different language servers and big files that require lots of formatting to make sure there are no catastrophic bugs
• Change default value of formatOnSave in config to false

[Andriamanitra]

I tested this with pylsp and a 10k line file that required bunch of formatting and was not happy with the results. With a big file like that it takes multiple seconds to get back a response and handle it. In that time you may easily edit the document without realizing the formatting is not yet done, which leads to the document getting out of sync.