Provide way to add pre-processing of data before ingest

[justinhayes]

In some cases it is useful to pre-process the text data before it is ingested. One example is to strip out specific pieces of text (e.g. boilerplate language that is on every webpage). Another example is to remove some formatting elements (e.g. if a webpage or a document includes html code in the body then you might want to strip that).

It would be useful to have a config option in the yaml file that lets you provide a list of find-and-replace regexes, like you can do in VIM to do a global replace. Then just before the data is ingested apply those regexes on (a) the text within each section, and (b) the title of the document and of the sections.