added ability to compare files with different number of columns

[KarolNi]

Implemented solution to #39 proposed by @tmtben

[DeflateAwning]

Please merge this request! This is a very useful feature!

[kevinp2]

@aswinkarthik appears to have gone inactive on GitHub.

@aswinkarthik, would you be able to merge this? Or consider giving the repository to another maintainer to carry on with?

[eslgastal]

The following test is not passing after applying this patch/PR:

csvdiff/cmd/config_test.go

Lines 244 to 272 in 1007bf3

	t.Run("should validate if positions are within the limits of the csv file", func(t *testing.T) {
	fs := afero.NewMemMapFs()
	{
	baseContent := []byte("id,name,age,desc")
	err := afero.WriteFile(fs, "/base.csv", baseContent, os.ModePerm)
	assert.NoError(t, err)
	}
	{
	deltaContent := []byte("id,name,age,desc")
	err := afero.WriteFile(fs, "/delta.csv", deltaContent, os.ModePerm)
	assert.NoError(t, err)
	}
	t.Run("primary key positions", func(t *testing.T) {
	_, err := cmd.NewContext(
	fs,
	[]int{4},
	nil,
	nil,
	nil,
	"json",
	"/base.csv",
	"/delta.csv",
	',',
	false,
	)
	assert.EqualError(t, err, "validation failed: --primary-key positions are out of bounds")
	})

--- FAIL: TestNewContext (0.00s)
    --- FAIL: TestNewContext/should_validate_if_positions_are_within_the_limits_of_the_csv_file (0.00s)
        --- FAIL: TestNewContext/should_validate_if_positions_are_within_the_limits_of_the_csv_file/inequal_base_and_delta_files (0.00s)
            config_test.go:327: 
                        Error Trace:    config_test.go:327
                        Error:          Error message not equal:
                                        expected: "base-file and delta-file columns count do not match"
                                        actual  : "base-file and delta-file columns count do not match and columns to selective compare not specified"
                        Test:           TestNewContext/should_validate_if_positions_are_within_the_limits_of_the_csv_file/inequal_base_and_delta_files