Multiple custom tag types

[JPinkney]

This PR finally brings in multiple custom tag types (which was heavily requested) into the YAML language server.

This PR relies on a custom fork of yaml-ast-parser hosted here that the only goal of it is to provide multiple custom tag types on top of the current parser. The reason why this is a custom fork of yaml-ast-parser is because:

1. The project is not really active anymore
2. The solution is specific to the YAML Language Server

Fixes:
#77
redhat-developer/vscode-yaml#142
redhat-developer/vscode-yaml#137

[coveralls]

Coverage decreased (-0.7%) to 71.006% when pulling 02e7587 on multipleCustomTagTypes into 498eaa2 on master.