Auto-generate Importer-Exporter markdown templates

[athityakumar]

As mentioned by @zverok in PR #64, the README seems to be quite crowded with examples and useful information of ALL Importer-Exporter modules. Rather, the corresponding links in the LOC in the README can be linked to corresponding module/{FORMAT}_IMPORTER.md. These module/{FORMAT}_IMPORTER.md can preferably be generated by a Rake task, via ERB templates.

[zverok]

In fact, my proposal was to have one doc/Importers.md file with all importers & autogenerated TOC. Source for it should/can be taken from lib/importers/importer_name.rb top comment block.