Resume existing index when appending to a HDFStore
When appending a DataFrame to a HDFStore it would be useful if there was a way to resume the current integer index of the existing data (and hence drop the index of new DataFrame).

A work around was posted [here](https://stackoverflow.com/questions/16997048/how-does-one-append-large-amounts-of-data-to-a-pandas-hdfstore-and-get-a-natural), but perhaps it would be better if there was a `ignore_index` flag in `HDFStore.append()`, a la `DataFrame.append()`.
