message: Add snippets for gather and multline environments
  I propose to add snippets for popular gather and multline environments:
  BGA for gather,
  BSGA for gather*,
  BMU for multline,
  BSMU for multline*
diff: "body": "\\begin{align*}\n\t$0\n\\end{align*}",
  "description": "Insert an align* environment"
  },
  + "gather": {
  + "prefix": "BGA",
  + "body": "\\begin{gather}\n\t$0\n\\end{gather}",
  + "description": "Insert a gather environment"
  + },
  + "gather*": {
  + "prefix": "BSGA",
  + "body": "\\begin{gather*}\n\t$0\n\\end{gather*}",
  + "description": "Insert a gather* environment"
  + },
  + "multline": {
  + "prefix": "BMU",
  + "body": "\\begin{multline}\n\t$0\n\\end{multline}",
  + "description": "Insert a multline environment"
  + },
  + "multline*": {
  + "prefix": "BSMU",
  + "body": "\\begin{multline*}\n\t$0\n\\end{multline*}",
  + "description": "Insert a multline* environment"
  + },
  "itemize": {
  "prefix": "BIT",
  "body": "\\begin{itemize}\n\t\\item $0\n\\end{itemize}",
  