Add continuous integration workflow

[TristanCacqueray]

This change ensures the library builds and fix some of the doc examples:

$ cargo test --doc 
   Compiling dominator v0.5.34 (/srv/github.com/Pauan/rust-dominator)
    Finished test [unoptimized + debuginfo] target(s) in 0.22s
   Doc-tests dominator

running 14 tests
test src/macros.rs - macros::props (line 682) - compile ... ok
test src/macros.rs - macros::clone (line 583) - compile ... ok
test src/macros.rs - macros::attrs (line 651) - compile ... ok
test src/macros.rs - macros::attrs (line 627) - compile ... ok
test src/fragment.rs - fragment::fragment (line 142) - compile ... ok
test src/macros.rs - macros::attrs (line 638) - compile ... ok
test src/macros.rs - macros::props (line 708) - compile ... ok
test src/macros.rs - macros::props (line 693) - compile ... ok
test src/macros.rs - macros::with_node (line 217) - compile ... ok
test src/macros.rs - macros::styles (line 763) - compile ... ok
test src/macros.rs - macros::styles (line 750) - compile ... ok
test src/macros.rs - macros::styles (line 739) - compile ... ok
test src/macros.rs - macros::apply_methods (line 132) ... ok
test src/fragment.rs - fragment::fragment (line 188) ... ok

test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.12s

[TristanCacqueray]

Hello @Pauan , would it be ok to run rustfmt on add a CI check too?