Allow adding the 'text' attribute to Option<String>

[ephraimkunz]

I use several of the same Rust structs for both JSON and XML serialization. I need certain properties in these structs to be optional for JSON serialization, but use the "text" attribute for XML. Previously this wasn't possible as yaserde didn't support the "text" attribute on anything but String and types that implemented Into<TokenStream>.

This PR adds support for serializing / deserializing the "text" of an XML tag into Option<String>.

Serialization

If an Option<String> is None, an empty string (the String Default impl) is written. If an Option<String> is Some, the inner string is written.

Deserialization

If the XML text is an empty string, None is produced. Else, Some<text> is produced.

[coveralls]

Pull Request Test Coverage Report for Build 276

• 14 of 24 (58.33%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 59.738%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
yaserde_derive/src/lib.rs	0	2	0.0%
yaserde_derive/src/ser/expand_struct.rs	0	3	0.0%
yaserde_derive/src/de/expand_struct.rs	0	5	0.0%
<!--	Total:	14	24	58.33%	-->

Totals
Change from base Build 272:	0.1%
Covered Lines:	1138
Relevant Lines:	1905

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💛 - Coveralls

[ephraimkunz]

Any ideas if / when someone could take a look at this PR? No huge rush, but I'm working on a library that depends on these changes and I'd like to avoid having to publish a fork of yaserde if possible.