new: added functions to convert with a number sequence as a word

[sttk]

This PR is for resolving #18.

In this PR, the new five functions are added for converting a string into each case with inserting separators before number sequences.

• cobol_case_with_nums_as_word
• kebab_case_with_nums_as_word
• macro_case_with_nums_as_word
• snake_case_with_nums_as_word
• train_case_with_nums_as_word

These functions can be used as follows:

use stringcase::snake_case_with_nums_as_word as snake_case;
snake_case("valueOf1");  // ==> "value-of-1"

In addition, this PR adds the new five methods of Caser corresponding to each function above.

• Caser#cobol_case_with_nums_as_word
• Caser#kebab_case_with_nums_as_word
• Caser#macro_case_with_nums_as_word
• Caser#snake_case_with_nums_as_word
• Caser#train_case_with_nums_as_word

These methods can be used as follows:

use stringcase::Caser;
"valueOf1".to_snake_case_with_nums_as_word();  // ==> "value-of-1"