Support markdown format on input (#1478)

* Support markdown on input

* unit-test files

* doc mods

* Unit-test cases for I/O-format keystroke-savers

* -i/-o md as well as -i/-o markdown

test/cases/io-format-conversion-keystroke-savers/c2p/cmd

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+mlr --c2p cat test/input/abixy.csv

test/cases/io-format-conversion-keystroke-savers/d2p/cmd

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+mlr --d2p cat test/input/abixy.dkvp

test/cases/io-format-conversion-keystroke-savers/j2p/cmd

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+mlr --j2p cat test/input/abixy.json

test/cases/io-format-conversion-keystroke-savers/j2p/expout

@@ -0,0 +1,11 @@
+a   b   i  x          y
+pan pan 1  0.34679014 0.72680286
+eks pan 2  0.75867996 0.52215111
+wye wye 3  0.20460331 0.33831853
+eks wye 4  0.38139939 0.13418874
+wye pan 5  0.57328892 0.86362447
+zee pan 6  0.52712616 0.49322129
+eks zee 7  0.61178406 0.18788492
+zee wye 8  0.59855401 0.97618139
+hat wye 9  0.03144188 0.74955076
+pan wye 10 0.50262601 0.95261836

test/cases/io-format-conversion-keystroke-savers/l2m/cmd

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+mlr --l2m cat test/input/abixy.json

test/cases/io-format-conversion-keystroke-savers/m2c/cmd

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+mlr --m2c cat test/input/abixy.md

test/cases/io-format-conversion-keystroke-savers/m2d/cmd

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+mlr --m2d cat test/input/abixy.md

test/cases/io-format-conversion-keystroke-savers/m2j/cmd

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+mlr --m2j cat test/input/abixy.md

test/cases/io-format-conversion-keystroke-savers/m2l/cmd

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+mlr --m2l cat test/input/abixy.md

test/cases/io-format-conversion-keystroke-savers/m2l/expout

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+{"a": "pan", "b": "pan", "i": 1, "x": 0.34679014, "y": 0.72680286}
+{"a": "eks", "b": "pan", "i": 2, "x": 0.75867996, "y": 0.52215111}
+{"a": "wye", "b": "wye", "i": 3, "x": 0.20460331, "y": 0.33831853}
+{"a": "eks", "b": "wye", "i": 4, "x": 0.38139939, "y": 0.13418874}
+{"a": "wye", "b": "pan", "i": 5, "x": 0.57328892, "y": 0.86362447}
+{"a": "zee", "b": "pan", "i": 6, "x": 0.52712616, "y": 0.49322129}
+{"a": "eks", "b": "zee", "i": 7, "x": 0.61178406, "y": 0.18788492}
+{"a": "zee", "b": "wye", "i": 8, "x": 0.59855401, "y": 0.97618139}
+{"a": "hat", "b": "wye", "i": 9, "x": 0.03144188, "y": 0.74955076}
+{"a": "pan", "b": "wye", "i": 10, "x": 0.50262601, "y": 0.95261836}

test/cases/io-format-conversion-keystroke-savers/m2n/cmd

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+mlr --m2n cat test/input/abixy.md