Add an option to automatically detect record type

## Background
Determining record type is critical for reading mainframe files. Yet, most issues raised are related to the confusion of specifying the type of records in a mainframe file. Simplifying this bit would help getting started with Cobrix quicker.

## Feature
Add an option, say,
```
.option("infer_record_type", "true")
```
that would read the input file and determine:
1. Presence and type of RDW (big-endian or little-endian)
2. True record size for fixed record-length files

Cobrix should log options used for reading such files so that the automatic inference could be replaced by these explicit options for future jobs.