Add support for hover extensions

[fwcd]

Fixes #69

This implements support for adding user-defined hovers via external commands, so-called extensions, which can be configured via the config (in VSCode settings.json):

{
  "curry.languageServer.extensions": [
    {
      "name": "Example Extension",
      "extensionPoint": "hover",
      "executable": "printf",
      "args": [
        "%s\n",
        "This is an example output:",
        "Current File:   {currentFile}",
        "Current URI:    {currentUri}",
        "Current Module: {currentModule}",
        "Line:           {line}",
        "Column:         {column}",
        "Type:           {type}",
        "Identifier:     {identifier}",
        "Module:         {module}"
      ]
    }
  ]
}

Other extension points may be added in the future.

To do

• Implement hover extensions
• Add some basic template parameters
  • currentFile, currentUri and currentModule
  • line and column
  • identifier and type (when local type information is available)
  • module (the imported module where the identifier is from, if available)