add col.types to duckdb_read_csv

[eli-daniels]

This addresses 2 issues: #118 and #383

col.types can be given as a named character vector providing the names and types of the columns such as: col.types = c(col0 = 'VARCHAR', col1 = 'COUBLE' , etc..) or an unnamed character vector, then col names are taken from the read.csv output or the col.names argument. Column names given by col.types are preferred over col.names.

The data types are provided as DuckDB data types, so VARCHAR, DOUBLE, BIGINT, etc...

As part of this I changed dbWriteTable to dbCreateTable. So could easily add the temporary parameter mentioned here #142, but don't want to get ahead of myself.

Also made a minor addition to the docs to mention that the csv files are appended to the table if the table already exists, I hope that is okay.

Happy to modify anything that may be amiss!