add rubocop

lua/guard/tools/linter/rubocop.lua

@@ -0,0 +1,33 @@
+local diag_fmt = require('guard.lint').diag_fmt
+return {
+  cmd = 'bundle',
+  args = { "exec", "rubocop", "--format", "json", "--force-exclusion", "--stdin" },
+  stdin = true,
+  output_fmt = function(result, buf)
+    local severities = {
+      info = 3,
+      convention = 3,
+      refactor = 4,
+      warning = 2,
+      error = 1,
+      fatal = 1
+    }
+
+    local offenses = vim.json.decode(result).files[1].offenses
+    local diags = {}
+
+    vim.tbl_map(function(mes)
+      print(vim.inspect(mes))
+      diags[#diags + 1] = diag_fmt(
+        buf,
+        tonumber(mes.location.line),
+        tonumber(mes.location.column),
+        mes.message .. " [" .. mes.cop_name .. "]",
+        severities[mes.severity] or 4,
+        'rubocop'
+      )
+    end, offenses)
+
+    return diags
+  end,
+}