Add a function to get all hyperlink in a worksheet

[undeflife]

currently, we can only get hyperlink cell by cell which is inefficient

rows, _ := e.GetRows(sheetName)
for rowNumber, row := range rows{
    for cellNumber, cell := range cols {
        cellRef := toCellRef(rowNumber, cellNumber+1)
        if ok, link, _ := e.GetCellHyperLink(sheetName, cellRef); ok {
        // do something to the link 
        }
    }
}

provide a function to get all external hyperlinks will enable a more flexible way to deal with hyperlinks

rows, _ := e.GetRows(sheetName)
links, _ := e.Hyperlinks(sheetName)
// k is a cellRef like 'A1' v
for k, v := range links {
  rowNumber, cellNumber := toCellPosition(k)
 fmt.Printf("cell %s : value is %s and has link to %s\n ", k, rows[rowNumber][cellNumber-1], v)
}

please comments on this and I am glad to open a pull request to implement it.