Add setting to customise delimiters

[thessem]

I would like to be able to customise the delimiters I use in templating (this is intended as an analog of https://golang.org/pkg/text/template/#Template.Delims).

This PR has the following problems

1. Delimiters is a byte array to limit them to single ASCII characters
2. The tag delimiters are formed using the objectLeft and objectRight argument, I would like to specify all 4 combinations completely, e.g., "<<", ">>", "<%", "%>".
3. I think moving compilation of the tokenMatcher regex into the function call will dramatically slow things down.
4. There are no tests.
   I did it like this so I didn't have to change the existing code much.

I am happy to continue work on this, but I'm lodging a PR to see if you're willing to accept this sort of functionality, if I'm going about implementing in the right way, and to check if I haven't missed some other locations where the delimiters are hardcoded.

[coveralls]

Coverage decreased (-0.08%) to 87.862% when pulling 9dd9191 on Proximaio:feature/customise_delimiters into de5fffa on osteele:master.

[osteele]

“delimeters” → “delimiters”

[osteele]

This looks like a reasonable way to do this. I added a benchmark to the project; it shows no performance penalty for this. Also using it on the Jekyll doc sources didn't show a measurable effect.

[I also tried to remove the AppVeyor check from CI. Since master doesn't pass, and has never passed, this is just adding noise.]

$ go test -bench=.  # before
BenchmarkEngine_Parse-8               30          45243520 ns/op
BenchmarkTemplate_Render-8           500           2304345 ns/op
PASS
ok      github.com/osteele/liquid       2.868s

$ go test -bench=.  # after
BenchmarkEngine_Parse-8               30          45490013 ns/op
BenchmarkTemplate_Render-8          1000           2339035 ns/op
PASS
ok      github.com/osteele/liquid       4.053s

[coveralls]

Coverage increased (+1.3%) to 89.289% when pulling dd4d967 on Proximaio:feature/customise_delimiters into de5fffa on osteele:master.

[thessem]

Please ignore that last commit, I'm editing it to support full strings instead of bytes right now, I'll post benchmarks when I think I'm done

[coveralls]

Coverage increased (+1.4%) to 89.308% when pulling b7ef67f on Proximaio:feature/customise_delimiters into de5fffa on osteele:master.

[thessem]

I added support for delimiters of any length. I had to mess up the regex expression building a little bit as the (?:\s+((?:[^%]|%[^}])+?)) part needed to be constructed based off whatever the custom delimiter was. I hope that I got the logic correct here as I had a little trouble parsing it.

% go test -bench=. -benchtime=10s -benchmem # before
BenchmarkEngine_Parse-4      	     500	  38626146 ns/op	11447556 B/op	   12103 allocs/op
BenchmarkTemplate_Render-4   	   10000	   2001773 ns/op	 1152073 B/op	   23020 allocs/op
PASS
ok  	github.com/osteele/liquid	43.451s

% go test -bench=. -benchtime=10s -benchmem # after
BenchmarkEngine_Parse-4      	     300	  39019683 ns/op	11463230 B/op	   12250 allocs/op
BenchmarkTemplate_Render-4   	   10000	   2060736 ns/op	 1184109 B/op	   23020 allocs/op
PASS
ok  	github.com/osteele/liquid	36.694s

If you are worried about the complexity of this I think it might be prudent to provide a default case with a hardcoded regex, so if the defaults are being used (as they are in 99.9% of cases) the hardcoded expression is used.

[thessem]

I am finished working on this PR unless you have any concerns that you'd like me to address.

[osteele]

Great, looks good to go!