[bashrc] `srch`: Processes file names with spaces

* Previously, when the `srch` function processed file names with
  spaces, grep would receive two file names, split at the space. For
  example, a file named "hello world.txt" would be given to the `grep`
  portion of the command as "hello" and "world.txt". Use the `-print0`
  option with the find command and the `-0` option with the xargs
  command to mitigate this: the former uses `NULL \0` to mark the end
  of a file name, the latter looks for `NULL \0` as a file name
  separator, thus sending the correct file name to grep.
* Added coloring via the `colouring=always` option for grep.
* grep recognizes extended regex commands. This should always be on.