Code generate TextLoader API and enhance it with convenience API. (dotnet#142)

*Code generate support for IDataLoader
*Make TextLoader API code generated so that it's at functional parity with the text loader in the ML.Net infrastructure.
*Move TextLoader API under Microsoft.ML.Data namespace
*Add convenience TextLoader API.
*Add error checking for invalid loader arguments such as ordinal, column names.
*Update baselines.
*Update samples with new loader API and backward compatibility with old loader API.

Author: codemzs

ZBaselines/Common/EntryPoints/core_ep-list.tsv

@@ -1,7 +1,8 @@
+Data.CustomTextLoader	Import a dataset from a text file	Microsoft.ML.Runtime.EntryPoints.ImportTextData	ImportText	Microsoft.ML.Runtime.EntryPoints.ImportTextData+Input	Microsoft.ML.Runtime.EntryPoints.ImportTextData+Output
 Data.DataViewReference	Pass dataview from memory to experiment	Microsoft.ML.Runtime.EntryPoints.DataViewReference	ImportData	Microsoft.ML.Runtime.EntryPoints.DataViewReference+Input	Microsoft.ML.Runtime.EntryPoints.DataViewReference+Output
 Data.IDataViewArrayConverter	Create and array variable	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro	MakeArray	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro+ArrayIDataViewInput	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro+ArrayIDataViewOutput
 Data.PredictorModelArrayConverter	Create and array variable	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro	MakeArray	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro+ArrayIPredictorModelInput	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro+ArrayIPredictorModelOutput
-Data.TextLoader	Import a dataset from a text file	Microsoft.ML.Runtime.EntryPoints.ImportTextData	ImportText	Microsoft.ML.Runtime.EntryPoints.ImportTextData+Input	Microsoft.ML.Runtime.EntryPoints.ImportTextData+Output
+Data.TextLoader	Import a dataset from a text file	Microsoft.ML.Runtime.EntryPoints.ImportTextData	TextLoader	Microsoft.ML.Runtime.EntryPoints.ImportTextData+LoaderInput	Microsoft.ML.Runtime.EntryPoints.ImportTextData+Output
 Models.AnomalyDetectionEvaluator	Evaluates an anomaly detection scored dataset.	Microsoft.ML.Runtime.Data.Evaluate	AnomalyDetection	Microsoft.ML.Runtime.Data.AnomalyDetectionMamlEvaluator+Arguments	Microsoft.ML.Runtime.EntryPoints.CommonOutputs+CommonEvaluateOutput
 Models.BinaryClassificationEvaluator	Evaluates a binary classification scored dataset.	Microsoft.ML.Runtime.Data.Evaluate	Binary	Microsoft.ML.Runtime.Data.BinaryClassifierMamlEvaluator+Arguments	Microsoft.ML.Runtime.EntryPoints.CommonOutputs+ClassificationEvaluateOutput
 Models.BinaryCrossValidator	Cross validation for binary classification	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro	CrossValidateBinary	Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro+Arguments	Microsoft.ML.Runtime.EntryPoints.CommonOutputs+MacroOutput`1[Microsoft.ML.Runtime.EntryPoints.CrossValidationBinaryMacro+Output]