Merge --path-to-cpp-types and --path-to-custom-types #130

Challenges

• I never know which is which...
• We support two types of specifying the built-in SQL types.
• I am always confused by the value, key {, key} format

Suggestion

Merge the two into one CSV with four columns:

1. What: column or type
2. Entity: A column name or an SQL type, depending on the What column
   Column names can be qualified with a table name but don't have to. In the latter case they match across tables.
3. C++ type: always fully qualified, e.g. sqlpp::integral or boost::uuid

Should CSV header be included? No.

Provide better unit test support to validate correct parsing and application of the CSV data.

Custom types probably require custom import or include directives. Add a parameter for that.