Support for formatting and header spell-check

Added support for formatting - in other words, bold text will be
highlighted in bold, italic text in italic and so on.

Enhanced spell-checking so that headings can be spell-checked. It doesn't
make much sense to me to spell-check paragraphs, but not headings. Text
within B<> and I<> can now also be spell-checked too. Again, it seems
strange not to spell-check text simply because it is bold or italic.

These features are controlled by separate variables: perl_pod_formatting
and perl_pod_spellcheck_headings which would normally be set from your
vimrc file. The features are disabled by default to maintain compatibility.
POD formatting escapes syntax highlighting improvements.

- Closing > characters are now highlighted correctly.
- Formatting escapes like B<>, I<> can now be used in POD commands like
  =head, =item and so on.
Fix perlElseIfError when used in other syntax type

When used in the Mason syntax type, the way the perlElseIfError group
was handled caused any line beginning with 'if' to be highlighted as an
error.  This is due to the way Vim includes other syntax files.
perlElseIfError was marked as 'contained', but wasn't limited to a
specific group.  When perl.vim gets included, perlElseIfError is then
allowed to exist at what was formerly the top level.

Instead, make the errant '\s*if' of 'elseif' and 'else if' a part of the
perlConditional group, so that perlElseIfError can be confined to
containedin=perlConditional.

In Perl files, everything that should be marked as a perlElseIfError is
handled properly.  In Mason files, this prevents the erroneous
perlElseIfError highlighting for 'if', but only marks 'elseif' as an
error (the :syn-keyword perlElseIfError works; the :syn-match doesn't).
Add minimal tests for tt2 syntax highlighting

Specifically test that b:tt2_syn_inc_perl works.  (It doesn't.)
Allow user to disable perl syntax inside of tt2

if it's set uncondtionally there's no need to check it later...
Set it as a default so that it can be turned off by custom vim scripts
highlight TODO: in addition to TODO

perl-support.vim plugin adds ':' to 'iskeyword' option.
At least, its older releases added it (can't find it in the latest
version). I like it because it allows me to move around using */# commands
quickly.
But if this option is configured this way, syntax/perl.vim doesn't
highlight 'TODO:' in comments, because TODO: is considered a complete
keyword.

Maybe I should've replaced 'syn keyword perlTodo' with 'syn match', but
it's not too much of copy-pasting (see the patch), and that's what
syntax/help.vim does for NOTE: lines in vim help files.
Merge pull request #84 from berekuk/highlight-todo-colon

highlight TODO: in addition to TODO
update credits
Words like FIXME and TODO now have their colon versions as well
make include path work for perl6
Merge remote-tracking branch 'petdance/master'
pathogen and vundle works for vim-perl
Changed the URLs to the new vim-perl organization
Need to use -[tT] switch if used on the shebang line
Merge pull request #95 from harleypig/master

efm_perl needs to use taint switch if used on the shebang line
Merge pull request #88 from eiro/master

my improvements
Replace tabs with spaces
Fix iskeyword to include dashes
Change keywordprg to p6doc for Perl 6
Fix up formatting for perlpath setter