Add support for es_if_exists='append' to pandas_to_eland()

[sethmlarson]

Adds support for appending a pd.DataFrame to a potentially existing Elasticsearch index.
The verification between mappings currently is pretty strict. All field and data types are compared
and only float types, integer types, date/date-nano, and keyword->text are allowed. We rely on Elasticsearch's own type checking for integer out-of-bounds issues. We're also currently requiring that every column exactly match every entry in the ES mapping, so they must be identical.

Open questions for me:

• Is being strict about columns/entries correct? Should we be less strict on one side?
• Am I missing any type compatibilities?
• Should I remove date-nano -> date since there is a loss of information?

Closes #173

[stevedodson]

LTGM!