Disambiguate .fr between Frege, Forth, and plain text files

This expands the scope of #1701, which is superseded if this PR is more appropriate.

There are about 20000 `.fr` files on GitHub.  They are currently all detected as `Frege`, but actually most are just plain text files (mostly in French).  This PR adds samples, heuristics, etc to vastly improve classification.

Tested with two sets of samples provided by @arfon: one with the keyword "the" and another one with "le".  Below are the result of running `linguist` **before** and **after** my changes.  **Actual** is my manual classification of the samples.

#### "The" samples:
| Language | Before | After | Actual |
|----------|-------:|------:|-------:|
| text     |        |   702 |    671 |
| Forth    |        |   132 |    132 |
| Frege    |   1000 |   122 |    122 |
| XML      |        |    44 |     49 |
| C        |        |       |     11 |
| Groff    |        |       |      8 |
| Makefile |        |       |      7 |

#### "Le" samples:
| Language | Before | After | Actual |
|----------|-------:|------:|-------:|
| text     |        |   910 |    916 |
| XML      |        |    41 |     41 |
| HTML     |        |    40 |     40 |
| Frege    |   1000 |     3 |        |
| Forth    |        |     3 |        |
| Shell    |        |     2 |      2 |
| Perl     |        |     1 |      1 |

Not that it will really matter but the correct translation is `la pomme de terre verte` ;)