Add suport for in-code defined SQL datasources

This PR adds the following features:

Execute multiple SQL statements
Create data sources using custom non-standard SQL statement (user don't need to create config file anymore)
octosql/ code and utilities to make Octosql more third-party friendly

Important notes:

A huge part of changes in this MR is sql.go file so the real number of changed lines is invalid. There's no sensible way I could divide this MR in two.

Detailed description

In my opinion, which may be biased, it's much quicker to copy just one (saved elsewhere) string containing SQL query than copying the configuration file and that string together. This PR introduces custom CREATE statements:

  CREATE DATASOURCE [name] OF TYPE [type] OPTIONS [options_specs]

Where [name] is valid SQL identifier, [type] is an SQL literal string containing type name of the data source and [options_specs] is either empty or is of the form:

 option_specs := ( <option_name> <sql_expr> [, <identifier> <sql_expr>...] )
 option_name := <identifier> | <indentifier> . <option_name>

Example usage:

  CREATE DATASOURCE bikes OF TYPE 'json' OPTIONS (path 'bikes.json');
  SELECT * FROM bikes bikes

Configuration keys can be nested if needed:

  CREATE DATASOURCE some_source OF TYPE 'some_type' OPTIONS (x.y.z 'value');

Please note: Nested values are supported, but arrays are not. The keys values are yet limited to SQL strings.

This is clean and expressive. The options notation was taken from SQLServer OPTION CLAUSE and entire syntax was based on CREATE SERVER

As the byproduct of that feature Octosql can now execute multiple queries in a single console command:

  $ octosql "SELECT * FROM bikes bikes; SELECT * FROM tests tests"

The other minor change was rewriting app/app.go into many files in octosql/ which exports very simple API. This way the code is much cleaner and let's third party tools (like native bindings) use awesome Octosql features much easier.

cube2222 / octosql

Add suport for in-code defined SQL datasources #175

Detailed description