add support for template output mode with new option --templage <tmpl> (#47)

t/test-template.txtar

@@ -0,0 +1,16 @@
+exec tablizer -r testtable.txt -P '{{.species | title}}s are {{.type }}'
+stdout 'Humans are invasive'
+
+exec tablizer -r testtable.txt -P '{{.species | sha256sum}}'
+stdout '79a5478768d2447431a90f7f4549df735f50ad541371464c248abc7522dc3a01'
+
+exec tablizer -r testtable.txt -P '{{add 100 .count}}'
+stdout 104
+
+-- testtable.txt --
+SPECIES   TYPE         HOME     COUNT  STAGE
+human     invasive     earth    4      brink
+riedl     peaceful     keauna   33     civilized
+namak     invasive     namak    123    imperium
+heduu     peaceful     iu       66     imperium
+kenaha    peaceful     kohi     3333   hunter-gatherer