feat: Get BAT_THEME var from environment (#200)

Closes: #196

lua/telescope/previewers.lua

@@ -23,6 +23,7 @@ local has_less = (vim.fn.executable('less') == 1) and config.values.use_less
 
 local bat_maker = function(filename, lnum, start, finish)
   local command = {"bat"}
+  local theme = os.getenv("BAT_THEME")
 
   if lnum then
     table.insert(command, { "--highlight-line", lnum})
@@ -40,6 +41,10 @@ local bat_maker = function(filename, lnum, start, finish)
     end
   end
 
+  if theme ~= nil then
+    table.insert(command, { "--theme", string.format("%s", theme) })
+  end
+
   return flatten {
     command, bat_options, "--", filename
   }