Refactor l:max_lines check in line iteration to be simpler

This makes the intent clearer
Make indentation of source code itself consistent

The need to do so is ironic.
Document the g:detectindent_max_indent setting
Fix inconsistent indentation in docs
Document my changes from the fork

So that people don’t think that *I* was the one who merged all semi-reasonable forks.

Also describe the state of the original plugin and say that it’s the one in the Vim plugin repository.
Make the `g:detectindent_preferred_expandtab` value be a boolean

… rather than assuming the variable is true because it exists, no matter its value

In other words, treat a `g:detectindent_preferred_expandtab` 0 value to signify a desired default of `noexpandtab`.

This allows the user to `let g:detectindent_preferred_expandtab = 0`, signifying that they definitely want `noexpandtab` if the detection is unsure. It makes the choice of the suggested “1” value in the documentation less arbitrary.
Extract similar-comment filetype lists into separate variables

And re-alphabetize the C-style comment filetype list.

Extracting them makes them easier to edit safely. It is also the first step in combining the lists so that it is easy to add even more comment types (such as using a map of filetypes to comment delimiters like NERDCommenter uses).
Change semicolon in verbose log messages to space

Before, a semicolon followed a period, which is ungrammatical. Now the semicolon is a space, which is grammatical, and also more clearly separates the variable list from the tab/space decision, which is a different type of information.