Better explanation/interface for ignoring strings

[matt-dray]

You can ignore certain strings when assessing cell values for their type. At the moment this requires the user to provide a regular expression, though there is a pre-filled default. A better approach might be to allow the user to input individual sets of characters without needing to provide it as a regular expression directly. There could be a series of tickboxes in the form 'ignore square brackets', etc, but I don't know all the possible things that people might want to ignore, so we should keep the input relatively 'open', I think.