Add kotlin lsp support, and :LspInstall kotlin (#263)

lua/lsp/kotlin-ls.lua

@@ -0,0 +1,36 @@
+--- default config for gradle-projects of the
+--- kotlin-language-server: https://github.com/fwcd/kotlin-language-server
+---
+--- This server requires vim to be aware of the kotlin-filetype.
+--- You could refer for this capability to:
+--- 	https://github.com/udalov/kotlin-vim (recommended)
+--- 	Note that there is no LICENSE specified yet.
+
+local util = require 'lspconfig/util'
+
+local bin_name = DATA_PATH .. "/lspinstall/kotlin/language-server/server/build/install/server/bin/kotlin-language-server"
+if vim.fn.has('win32') == 1 then
+  bin_name = bin_name..".bat"
+end
+
+local root_files = {
+  'settings.gradle',       -- Gradle (multi-project)
+  'settings.gradle.kts',   -- Gradle (multi-project)
+  'build.xml',             -- Ant
+  'pom.xml',               -- Maven
+}
+
+local fallback_root_files = {
+  'build.gradle',          -- Gradle
+  'build.gradle.kts',      -- Gradle
+}
+
+require'lspconfig'.kotlin_language_server.setup {
+    cmd = {bin_name},
+    on_attach = require'lsp'.common_on_attach,
+    root_dir = function(fname)
+      return util.root_pattern(unpack(root_files))(fname) or
+      util.root_pattern(unpack(fallback_root_files))(fname)
+    end
+}
+