Precompile sakefile

Precompiling is useful for writing **Sakefile** in various compile-to-JS languages.

Precompiling is performed by JavaScript function from region embedded in **Sakefile** and bounded by required marker lines. Marker line format is inspilred by Vim modeline (but it's different).

Function region (along with marker lines) is suited to be located in comments of host language:
* marker lines are independent of its prefixes,
* prefix of start marker line is stripped from every following  function source line.

Function must take one argument (string with **Sakefile** source) and return string with compiled source.

PR contains further restrictions to general idea:
* only files without extension are searched for function region,
* start marker must be located in first 5 lines of **Sakefile**,
* function length is limited to 100 lines.
* format of marker lines:
  - start marker line: `<prefix><white>sakefile:<white*>{`
  - end marker line:   `<prefix><white>sakefile:<white*>}`
  - where `<prefix>` -- any characters,
        `<white>`  -- one space or tab character,
        `<white*>` -- zero or more spaces or tab characters