Keep default conceal values for excluded files

Hello Yggdroot,

Had a little issue with viewing latex files even thought they were part of ```g:indentLine_fileTypeExclude```. I changed a few lines of your plugin to accommodate such a situation. 

Setup will now check if the file is part of the included or excluded filetypes before changing the conceal values. I basically moved them from the  ```if !&hidden || !exists("b:indentLine_set")``` block to beginning of the Setup function. This will allow latex files to be displayed correctly fixing issue #78, without completely disabling the plugin.

Let me know what you think