New function: `SearchSheet()`, relate issue #277

Fix the issue caused by missing tradition to strict conversion for `sharedStringsReader()`, relate issue #276

Add Support for WriteTo Buffer

* add new exported function to get Excel file content buffer

* refactor the WriteTo function

Update documentation and add sample file

data validation drop-down list use sqref cell  issue #268

 Initialize data validation support

Change `allowBlank`, `ShowErrorMessage` and `ShowInputMessage` type as boolean, add new field `ShowDropDown`, change fields orders follow as ECMA-376-1:2016 18.3.1.32.

Issue templates have been added.

Use min/mid/max value for 2 and 3 color scale conditional formatting

added retrieval of worksheet comments

- RGBA, HSL color convert has been added;
- go test updated

- Fix doc typo

Extract WriteTo method (see io.WriterTo) to expose bytes written