Allow commas in headers

[bartero]

First of all thank you for keeping up this library project.

The motivation behind this PR is to solve the problem https://github.com/ironSource/parquetjs/issues/117 In one sentence this PR extends the capability of this library to support commas in schema headers :-)

As a by product also included:

• the code in the few functions (findField, findFieldBranch, materializeRecords) seems to be somewhat simpler (in my reckoning at least)
• the performance of these functions should be improved (though have not benchmarked it)

I tried to be careful not to impose any unexpected side effects. I extended the read/write tests to cover the new case. I hope this PR meets your standards. I am open for discussion and criticism.

With regards, Bartłomiej