Basic PlantUML highlight support, issues wih '<' character.

[yangwenbo99]

1. The problem with the < character still appears on my system. This is due to the problem in the syntax file shipped with my vim installation. To make the behaviour consistent in different environment, it is better to use a modified HTML highlight file.
2. I also added basic highlight support for PlantUML blocks:

@startuml

class context {
	- state: State
	+ request
}

interface state {
	+ handle
}

Context <>-- state
state <|-- State1
state <|-- State2

@enduml

[shirosaki]

Travis CI tests fail with HTML tag in text.
Please run make test.
https://github.com/plasticboy/vim-markdown/blob/691d961a643064ee1feddba8371ff12e44678f5a/test/syntax.vader#L1269

[tpo]

Duplicate of #138