Support dedent with default indent

[sudo-suhas]

This adds a higher-order function makeDedent to allow creation of string tags with default indent. This is useful if all our lines are equally indented but we want to be able to specify an indent for all lines. Right now, using dedent strips the indentation from all lines. There is no change in the behavior of the default string tag. I haven't yet updated the readme, will do if this PR can be accepted.

P.S: The irony of supporting indentation in a repo called dedent is not lost on me.

[sudo-suhas]

@dmnd, would you be willing to accept this PR? I can either close it or do the required changes. Thanks