Use new API for textDocument/documentSymbol #407

Hi,

Since VScode 1.25's new outline feature (see redhat-developer/vscode-java#586),
the LSP spec has been updated (https://microsoft.github.io/language-server-protocol/specification#textDocument_documentSymbol) with new features, the most important one being the children? field, which allows for a hierarchical outline to be created from the symbols returned, will python-language-server be implementing this new spec?