v0.5

• Added support for subgraphs and Gantt charts in mermaid diagrams
• Added function graphviz_nodes_edges_df for generating a Graphviz DOT code that defines nodes and edges (and their attributes) from data in two data frames: one for nodes, the other for the edge operations
• Added function graphviz_single_df for generating Graphviz DOT code from a single data frame
• Incorporated the new substitution operators @_{...} or @^{...} in grViz statements for subscripting and superscripting, respectively