Allow the conversion of booleans to numbers

[lcorbasson]

When a table has a column with only 0 and 1 as values, the agate TypeTester detects a Boolean column. If later in a script we want to merge this table with another (let's say the same dataset for another year) where the very same column has other numbers (detected as a Number column), we need to convert the Boolean column to Number.

To do it properly in user code, we need to write a dictionary or a function, to take into account 0/False, 1/True and None. This is tiresome and subject to errors (when users might forget None; also when raising exceptions which won't be agate CastErrors in this case).

This change would allow an easy change of column type: we would just need to change the data type of the column to cast the values correctly.

This is also coherent with Python 2 and 3 having bool as a subclass of int (see e.g. here).

[jpmckinney]

Looks good! Can you add a test?

[lcorbasson]

Done, thanks!