mindr v.1.2.0 released: universal function and directory tree

The new version 1.2.0 mainly brings four exciting features.

1. Suggested by @yihui, an argument of 'method' was added to each main functions. Users can choose the method of regular expression or pandoc to pick out the outline of a markdown file.
2. Suggested by the issue from the users, now mindr can save the mind map as an HTML widget file and share the mind map on web.
3. A new function tree() can create a mind map from a directory on your computer. See the following image as a demo.
4. A universal function mm() can convert the file formats automatically according to the suffix of the input file. This is a big step of mindr, because mm() wraps (almost) all the previous features into one function. Users do not have to memorize other functions.