task.lua: remove special handling of default task names
task.lua: remove special handling of default task names

There was code which had no significant behavior other than to prevent
the default task name from matching a pattern.   In practice the
prevented behavior seems useful, as in the example brought up in #87.

    local task = require('lark.task')

    task.default  = 'stl/keyboardplatform.stl'

    stl = task.pattern[[^stl/.*%.stl]] .. function(ctx)
        local output = task.get_name(ctx)
        local src = string.gsub(output, '%.stl$', '.scad')
        src = string.gsub(src, '^stl/',  'src/')
        lark.exec('openscad', '-o', output, model)
    end

bmatsuo/lark,bmatsuo/lark