Rework customization to use lsp-mode's functionality

[michaelpj]

This does two things:

• Create defcustom variables for settings, matching the vscode
  extension for all the language server settings.
• Use lsp-mode's support for custom settings to handle sending them to
  the server.

This lets users persistently configure their server settings using
normal Emacs setting customization.

This will probably break people, so should maybe be a major version
bump.

Fixes #69, #75, #78; supersedes the (excellent) #74 and #76.

Notes:

• I have not copied the settings from the vscode extension regarding
  starting the server. We don't try and get prebuilt binaries, so I
  thought it was simplest to stick to command-and-arguments.
• I renamed the process option for consistency with other servers and to
  avoid references to hie. This PR will already break basically everyone, so
  I thought I might as well do that too.
• The current customization functions re-send a didChangeConfiguration
  notification when called. lsp-mode does not currently do this when
  you change variables, which is annoying. My inclination is to let them
  fix it in the name of simplicity, but if anyone really hates not being
  able to change the formatter without restarting the server I can try and
  hack something together.

[shaunplee]

@michaelpj Thank you for the kind word. I'm glad these issues got addressed and merged.