Report error context with a customisable length

[MaxDesiatov]

As reported in CoreOffice/CoreXLSX#12 there are multiple issues where it's hard to understand why parsing of a certain XML has failed. XMLCoder should be able to report a snippet of a given XML of specified length around the position where the error has happened. This should be an property of type Int? named errorContextLength on XMLDecoder instance. An error thrown from XML parser that has line/column location should be repackaged having a context around that location of specified length.

For example, if an error was thrown indicating that there's an unexpected character at line 3, column 15 with errorContextLength set to 10, a new error type should be rethrown containing 5 characters before column 15 and 5 characters after, all on line 3. Line wrapping should be handled correctly too as the context can span more than a few lines.

[MaxDesiatov]

Reopening this after a few bugs were reported in a corresponding PR.