Fix panic in Set Delimiter parsing.

[fiirhok]

Mustache supports Set Delimter tags, of the form {{=<open tag> <close tag>=}}, which is used to change the delimiters from {{ and }}. Prior to this commit, if there was a sequence that looked like the start of a Set Delimiter tag, but there was not a complete Set Delimiter tag, the library could panic.

This commit:
* factors out the Set Delimiter tag parsing into a new function, 'parseCustomDelimiter()'
* adds checks to make sure an incomplete tag doesn't cause an access off the end of a slice
* treats an incomplete Set Delimiter (like {{=}}) as a normaal tag

This behaviour matches the mustache playground at
https://jgonggrijp.gitlab.io/wontache/playground.html. I tried to match the behaviour of the Ruby implementation, but it seems to be inconsistent in this case.

[cbroglie]

Thanks for this @fiirhok. It looks like there's a failing test:

--- FAIL: TestBasic (0.00s)
    mustache_test.go:222: "{{ a }}{{=<% %>=}}<%b %><%={{ }}=%>{{ c }}" expected "abc" but got error "line 1: empty tag"
    mustache_test.go:222: "{{ a }}{{= <% %> =}}<%b %><%= {{ }}=%>{{c}}" expected "abc" but got error "line 1: empty tag"
panic: runtime error: slice bounds out of range [1:0] [recovered]
	panic: runtime error: slice bounds out of range [1:0]

[fiirhok]

Whoops, I'm not sure how I missed that. I added a commit that should fix things.