Option for setting custom url for LanguageTool server

[hesstobi]

This pull request add this features:

• allow to define a custom url for the LanguageTool server (e.g. if the server is running as local service)
• start and stop the user defined server on package activation and deactivation and on configuration changes (e.g. change of the config file path) - Fixes (#19)
• lint only the document if the server is reachable
• if the local server can not be started or the custom url is not reachable the linter falls back to the public server
• shows the version and build date of the server in the status bar
• option to set the port of the local server
• some tests for the local server for osx

[davidlday]

:+1: Good work, @hesstobi !

[wysiib]

Took a moment to read through all the changes. I agree, good work! Thanks you for your contribution. I already prepared a new release 0.7.0.