add char/page limit

[xiaoxin01]

Large file will cause memory leak, so add char/page limit function.

[xiaoxin01]

@jonathaningram Could you please help to review this PR?

Because some pdf or docx files have many pages and take a lot of time to parse text, I think a limitation is needed.

[jonathaningram]

@xiaoxin01 thank you for the PR. We'll take a look over it and get back to you as soon as possible. This PR changes the API surface of the package a fair bit so we'll want to look into those changes and we'll discuss here.

[jonathaningram]

@xiaoxin01 I think the best approach for supporting this kind configuration is to introduce new Converters. Something like:

package docconv

type PDFConverter struct {
  pageRange []int
  maxWords int
}

func NewPDFConverter(pageRange []int, maxWords int) *PDFConverter {
  return &PDFConverter{
    pageRange: pageRange,
    maxWords: maxWords,
  }
}

func (c *PDFConverter) Convert(r io.Reader) (string, map[string]string, error) {
  // ...
}

We want to avoid anymore global package state so I think a solution like this is probably going to be better. What do you think?