Set default text width in vim to 76 characters

And here come the rationale for this sensitive topic:
  - Source code should wrap at 80 characters; be polite and give room for
    line numbering. Four characters is room for up to 999 lines, followed by
    a space; if the file exceeds that length, it is probably difficult to
    work with, because it's hard to keep that much in working memory.
  - Allows for up to three levels of nesting for e-mail quoting (">>> "). I
    really hope more than that is not needed. If it is, modern editors can
    easily reflow text (e.g. `gq' in vim).