Add exponent-letter D to Fortran lexer

In the Fortran specification, the exponent-letter within a
signed-real-literal-constant is E or D.  Therefore, both 3.14159e0 and
3.14159d0 should be classified as a Number.Float.

The current behavior is

>>> print(highlight("3.14159d0", FortranLexer(), HtmlFormatter()))
<div class="highlight"><pre><span></span><span class="mf">3.14159</span><span class="n">d0</span>
</pre></div>

where the only the significand is classified as a Number.Float while
the exponent-letter and exponent are classified as Name.Variable.

After this patch the behavior is

>>> print(highlight("3.14159d0", FortranLexer(), HtmlFormatter()))
<div class="highlight"><pre><span></span><span class="mf">3.14159d0</span>
</pre></div>

which is the expected behavior.

0 files changed, 0 insertions, 0 deletions