starlark: dedent continued function param lines

[alexeagle]

This allows multi-line parameter documentation under Args: without extra indents.
Fixes bazelbuild/stardoc#74

[alexeagle]

ping @c-parsons are you the right person to merge?

[c-parsons]

Could you modify pure_markdown_test as well to demonstrate how this change affects markdown tables?

[c-parsons]

We're not truly finding the minimum indentation value here, as we're still processing as we go.
Consider the difference in behavior for:

  Line one
          Line two
                    Line three

versus

               Line one
        Line two
   Line three

[alexeagle]

correct, but the failure mode is that you get some extra whitespace if the continuation is accidentally not consistent. For your example I think

Line one
        Line two
                  Line three

and

Line one
Line two
Line three

are the best outcome we'll get without scanning ahead for the minimal indent. Do you think it's worth adding the complexity to scan ahead?

[c-parsons]

Yes, I don't think it's much more complex to scan ahead (we're not dealing with gigantic strings here, so it should be fine to store all the lines in a list, take the min indent, and then iterate over)

I foresee later user confusion / frustration with the current behavior, if we don't fix this right the first time.

Thanks!

[alexeagle]

Okay, done PTAL

[c-parsons]

FYI @alandonovan @brandjon

[alexeagle]

Added the change to pure_markdown_template_test as requested :)

[c-parsons]

FYI @alandonovan

[alexeagle]

ping @c-parsons mergey merge-merge?

[c-parsons]

Merge in review. Thanks for your patience.