Added macros for conveniently creating `smart-tabs-insinuate-alist` entries.

- `smart-tabs-create-language-advice`: Given a language symbol, a mode-hook,
    an (indent-function . offset) advice alist and an optional body, create
    a cons cell as was present in `smart-tabs-insinuate-alist` but much
    less verbosely. It fills in much of the boilerplate code.

- `smart-tabs-create-advice-list`: Given an (indent-function . offset) advice
    alist, construct a `(smart-tabs-advice indent-function mode-hook) list.

- `smart-tabs-when`: Can be used in the body of
    `smart-tabs-create-language-advice` to only add advice if some condition
    holds; for example `(smart-tabs-when (featurep 'python-mode) advice-list)`.

Modified definition of `smart-tabs-insinuate-alist` to use new macros.