* Implement consistent row addressing by Excel row number starting with 1

1. Added second versions for all row manipulation methods with zero-based row addressing.
2. Fixed methods documentation to explicitly describe which row addressing used in method.
3. Added WARNING to README.md.
4. Cosmetic change: All row test moved to file `rows_test.go`.

* TravisCI: go1.12 added to tests matrix

* BACKWARD INCOMPARTIBLE: Use only Excel numbering logic from 1 row

* README updated

DuplicateRowTo() is similar to DuplicateRow() but copies specified row not just after specified source row
but to any other specified position below or above source row.

Also I made minor modifications of tests: using filepath.Join() instead of direct unix-way paths strings
to avoid possible tests fails on other OS.

* #195: proposed resolution to the issue

* Make GetRows return value avoid empty cell

* Update test file to fix broken testing.

* go mod tidy applied

* File.DuplicateRow() method added

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

- Add error return value for functions: `AddChart()`, `AddComment()`, `AddPicture()`, `AddShape()`, `AddTable()` and `SetConditionalFormat()`
- go test has been updated

- go test and godoc has been updated

- go test and godoc has been updated

Signed-off-by: Ri Xu <xuri.me@gmail.com>

Signed-off-by: Ri Xu <xuri.me@gmail.com>

- API changed, use worksheet name instead of "sheet" + index, related issue #25, #43, #47, #51, #89, #101, #116 and #120.
- go test updated

- Readme and go test updated

Function `GetCellValue()` performance improvement by avoid repeating deserialization, relate issue #70.

- Unify the index row number index of functions `SetRowHeight()` and `GetRowHeight()` relate issue #68;
- Unify the return value data type of functions  `SetColWidth()` and `GetColWidth()`;
- go test updated

- Make function `TitleToNumber()` exportable, note that function `ToAlphaString()` return value calculation changes, get more info from go doc. Relate issue #63;
- Readme and go doc updated

- go test updated

- Function `SetCellValue()` support `time.Time` data type parameter, relate issue #49;
- go doc and go test updated

- Function `GetRows()` doc updated, relate issue #43;
- go test and embed template updated

- Doc for function `GetRows()` updated, relate issue #43;
- Fix doc typo in `xmlContentTypes.go`;
- Default template updated;
- Readme updated;
- go test updated

- Partial logic performance optimization, use pointer reference instead of a pass the variable value;
- Add comments for content types struct definition;
- Update go test `TestSetBorder` section