Fix: Use "filterText" (fallback to "label") for the trigger of the completion item

[rwols]

An option is added to always prefer "label" over the "filterText" if the user so desires. However, certain language servers might use the "label" to denote extra data, e.g. a circular dot in front of the word-to-complete in case it returns a completion with an "additionalTextEdits" key. In this case, using the "label" for the ST trigger characters can give wrong behavior. An example of this wrong behavior is: #368

One can take a look at the completion_test.py file to get a feel for what this change entails.

ping @ayoub-benali @frou

[coveralls]

Coverage increased (+0.006%) to 26.705% when pulling 495fb92 on bugfix/completions into bcada7c on master.

[ayoub-benali]

However, certain language servers might

Another argument to have the "label" actually the default and using the "filterText" as the exception. i.e. I would flip default of prefer_label_over_filter_text flag

[tomv564]

I accept the change of default behaviour although based on the premise that:

We cannot do something useful with 'dressed up' label content in Sublime Text,
and language servers can be expected to send fairly 'plain' content in the filterText field.

An expected issue is with completions with leading $ characters, would be great if someone tested a PHP language server with this behaviour.