Allow setting custom languagetool server startup command

[ipvych]

This is my attempt at fixing #6, this adds new variable flymake-languagetool-server-command that is used to launch server. I tried preserving default behavior by allowing symbols to be evaluated in that customization option and setting it to previous command

Some other changes:

• I removed flymake-languagetool-server-args, since now user can just modify command directly
• I removed flymake-languagetool--started-server my thought is that since we check if server process is live when starting it anyway this variable is not really needed, and it may confuse some users since if server stops unexpectedly user cannot relaunch server again until this variable is set to nil
• I replaced check when starting local languagetool server to check whether flymake-languagetool-server-url is set, since flymake-languagetool-server-jar may not be always set now if user decides to just modify server command

[ipvych]

I have not tested it with remote server and local server started using java since my main goal is to allow starting server using shell script provided by my distro. Please report if you have issues starting server using one of those methods.