Add new str_dedent function

[chrimaho]

One incredibly helpful function in Python is the textwrap.dedent function. Under the hood, this function uses regex to strip any leading spaces, while maintaining any internal indentation within a chunk of code.

This addition here re-implements the same functionality using native R code.

I've ensured to include 4 different unit tests for the same.

[arnaudgallou]

I'd also like to have a dedent() function in R. Two comments:

• Wouldn't it make sense to trim leading and trailing whitespaces in the output or add an argument to do so? That would be very useful when making character strings from multi-line text in R:

  str_dedent("
    This is a long sentence that starts on the first line,
    continues on the second,
    and ends on the third one.
  ")

  For readability purposes, it's better to start the text on its own line here. We wouldn't want to keep that blank line at the beginning though. Passing the output to str_trim() every time in that situation would be cumbersome.

• The function currently doesn't support the following situation (when python's textwrap.dedent() does):

  str_dedent("
      foo
    bar
  ")

  It's probably uncommon enough not to support it but that could be detailed in the function documentation.