Pros:
- it is convenient to type and read numbers this way;
- Ada, Perl, C++, Go and Rust already support that;
- can support thousands as well as lakhs and crores;

Cons:
- standard SQL does not have this support;
- PostgreSQL treat `SELECT 1_000_000` as number one with `_000_000` alias: http://sqlfiddle.com/#!17/0a28f/400
- C++ has `1'000'000` instead of `1_000_000`;

What do you think?

#include <base/defines.h>
#include <Parsers/Lexer.h>
#include <Common/StringUtils/StringUtils.h>
#include <base/find_symbols.h>
namespace DB
{
namespace
{
template <char quote, TokenType success_token, TokenType error_token>
Token quotedString(const char *& pos, const char * const token_begin, const char * const end)