Add support for multiple money columns

[oskarth]

This is useful in cases where there are two money columns and this can't easily be automatically detected.

• Adds command line option for --money-columns
• Updates parser to deal with 1-2 specified money columns
• Keep current money_column option for backwards compatibility
• Adds integration test case for this

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Things to note:

• Tested with ./spec/integration/test.sh spec/integration/two_money_columns_manual

• Running > reckon -f spec/integration/two_money_columns_manual/input.csv --unattended --account Assets:Bank:Checking results in Using column 3 as the money column. Use --money-colum to specify a different one.. Possibly due to negative money score (?). Same with --ignore-column 5

• I'm not a Ruby dev so code might be unidiomatic in some way.