Unifying Read & Write Interface

[asarrabay]

Currently we have asymmetric interface between read and write tabular data :
Read :

File(pathTo("simple_data.xlsx")).inputStream().use {
        val columns = object:RetableColumns() {
            val FIRST_NAME = string("first_name", index = 2)
            val LAST_NAME  = string("last_name", index = 1)
            val AGE        = int("age", index = 3)
        }
        val retable = Retable
                            .csv(columns)
                            .read(it)
}

Write :

        val columns = object:RetableColumns() {
            val FIRST_NAME = string("first_name", index = 2)
            val LAST_NAME  = string("last_name", index = 1)
            val AGE        = int("age", index = 3)
        }
        Retable(columns)
            .data(
                    listOf(
                            Person("John", "Doe", 23),
                            Person("Jenny", "Boe", 25)
                    )
            ) {
                    mapOf(
                            FIRST_NAME to it.firstName,
                            LAST_NAME to it.lastName,
                            AGE to it.age
                    )
            }
            .write(Retable.excel(columns) to File(pathTo("export_data_cols.xlsx")).outputStream())

It's would better to have same code to read and write a file until we call read or write method.