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
Check if perl6 is executable before calling it
Merge pull request #83 from rwstauner/tt2-disable-perl

Allow disabling perl syntax in tt2 syntax
Add folding of anonymous subs (Perl 5)

This is off by default; set perl_fold_anonymous_subs to
a truthy value to turn it on