Store tags in one big pre-allocated string and only work with refs

This should decrease the number of allocations before main is entered
significantly, but complicates the code somewhat in return.

Assuming I used `massif` right, doing just `SelfTest --list-tests`
went from 929 allocations at "Remove gcc-4.9 from the travis builds"
(2 commits up), to 614 allocations with this commit.

include/internal/catch_test_spec.cpp

@@ -43,9 +43,11 @@ namespace Catch {
     {}
 
     bool TestSpec::TagPattern::matches( TestCaseInfo const& testCase ) const {
-        return std::find(begin(testCase.lcaseTags),
-                         end(testCase.lcaseTags),
-                         m_tag) != end(testCase.lcaseTags);
+        return std::find_if(begin(testCase.tags),
+                            end(testCase.tags),
+                            [&](Tag const& tag) {
+                                return tag.lowerCased == m_tag;
+                            }) != end(testCase.tags);
     }
 
     bool TestSpec::Filter::matches( TestCaseInfo const& testCase ) const {