Add a new page about the printf command

- printf is a bash command that acts like C's printf, supporting format
   strings and modifiers.

Signed-off-by: Jose Ricardo Ziviani <jrziviani@gmail.com>

pages/common/printf.md

@@ -0,0 +1,23 @@
+# printf
+
+> Format and print text
+
+- Print a text message
+
+`printf {{"%s\n"}} {{"Hello world"}}`
+
+- Print an integer in bold blue
+
+`printf {{"\e[1;34m%.3d\e[0m\n"}} {{42}}`
+
+- Print a float number with the unicode Euro sign
+
+`printf {{"\u20AC %.2f\n"}} {{123.4}}`
+
+- Print a text message composed with environment variables
+
+`printf {{"var1: %s\tvar2: %s\n"}} {{"$VAR1"}} {{"$VAR2"}}`
+
+- Store a formatted message in a variable (does not work on zsh)
+
+`printf -v {{myvar}} {{"This is %s = %d\n" "a year" 2016}}`