0.1.16

• BREAKING: allow basic expressions in variables and task commands
  This is using the evalexpr crate. Expressions are used make-style
  by wrapping them in $(), e.g., $(1+2).

  This requires previous $(echo foo) meant to be subshells to be escaped:
  $$(echo foo).

• implement task required_vars
  This allows task availability depend on the existence of global variables.

• cli: drop "tasks" cli subcommand, add tasks to "build"
  Instead of laze task ... <task-name> [<task args>], just do
  laze build ... <task-name> [<task args>].

• cli: improve "build" subcommand help message