v4.18.1 - New features, more consice usage

• eval is now the default command, you can leave it out 👯 #113
• - no longer needs to be specified as STDIN, (unless you are also working with multiple files) 👯 #113
• Adding to empty maps / arrays now uses idiomatic yaml styling by default
• Fixed seg fault on bad input #1086
• New envsubst operator! (thanks @sciyoshi)
• Added support for *=, relative multiply/merge
• Custom tag types now autocast to there actual types #933