Watch mode

[edemaine]

• glob support like svgink
• -w command-line option to watch input files for changes and then recompile them (with appropriate preceding mappings).
• Detect dependencies in input files (e.g. CoffeeScript file imports another file) and update when those change too.
• Track which symbols got resolved from what mapping file, to know which mapping files a drawing actually depends on. (Maybe unnecessary.)

Related to #70 which also needs to compare dates. With #70, don't need to worry about detecting changes too often, because we can see nothing changed on the SVG side.