Creation of prettytable-rs-derive and the creation of table from vector

[thymbahutymba]

Pull request referring to the issue #109

Sorry for the mess that I've made trying to improve the TableElem method removing &self from get_field_name
The code is already rebased. This should be a kind of final code which is the results of the previous commit and pull request #110

[codecov]

Codecov Report

Merging #115 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   85.86%   86.15%   +0.29%     
==========================================
  Files           5        5              
  Lines        1323     1351      +28     
==========================================
+ Hits         1136     1164      +28     
  Misses        187      187

Impacted Files	Coverage Δ
src/lib.rs	88.19% <100%> (+0.64%)	⬆️

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[thymbahutymba]

I'm also wondering, do you want a PR that formats the code with rustfmt?

[thymbahutymba]

I was thinking about to make more generic the from_vec method in such way

pub fn from_vec<T, E>(v: T) -> Self 
	where
		T: IntoIterator<Item = E>,
		E: TableElem,

I'm not an expert of Rust so I'm asking for your feedback because you may noctice some trouble that may appear in such cases.