Fix highlighting indentation on lines starting with HTML.
Some Pygments highlighters mark up each line with a span tag, which the

indentation highlighter would nibble on as it tried to highlight the

indentation.
It's now smarter, and understands that it may have to share with other

span tags living on that line. If it encounters a span tag as the first

character, it will start looking for the spaces inside, nesting further

if needed.
If it can find all the whitespace it wants to show indentation for, then

it will do so. If it can't (for example, if certain sequences of spaces

were wrapped in a span, but not all), then it will bail and just not

show anything, to be safe.

Tested with the .rst file we first noticed this on. Pygments had begun each

code block line with a <span class="s">, which was no longer being eaten.

Instead, the indentation span was placed inside and properly wrapped the spaces.
Wrote unit tests to test this and to test for unexpected characters in the

assumed range of indentation. They pass.