Issue with invalid csv column names

[jonwarghed]

Having issue with that my csv column names are not valid bigquery names. Renaming and handling this is outside my control and schema is updated frequently. Ran into this library which makes it much easier but noticed I had to process everything twice to clean up the invalid names.

Bigquery does an automatic substitution in accordance with what is described in this issue #14

So added a pull request that allows one to run it with a sanitize names mode if wanted.