Infer options.useTabs when not specified

It would be awesome if recast inferred `options.useTabs` from the file contents, just like it does with `options.tabWidth` right now.

Right now, in a tab-indented file with `useTabs` not set to true, newly injected statements will be space-indented (using, I believe, a 4-space tab width by default). It would be great if `useTabs` were inferred as being true for newly injected statements in that case.

Start by tracing the existing tabWidth inference in recast and identify where indentation for newly injected statements is selected. Check how file contents indicate tab indentation. Done means omitted options.useTabs follows the input file's indentation style, with coverage for tab-indented files.

Written by the indexing model from the issue text.